I was curious about this law, so I spent some time and parsed through the text in the link Roy provided.
First, it seems pretty clear to me that this article has been translated from Portuguese or was written by someone who is a native Portuguese speaker. That makes an already complex law just a little bit more difficult to understand. Certain words are used repeatedly in the article to mean different things, so parsing which meaning is being used at any given point is difficult.
Second, it does seem overly complicated to me. After parsing each section I think I was able to figure out why it is included, but some of the sections seem like they're in place to cover some pretty small corner cases.
One caveat - I'm just going to say "produced" when in fact the law says "produced or acquired". I'm lazy and don't want to have to type that over and over.
In plain English, with certain exceptions based on this year's production, as long as you produced this year at least 75% as much Port as you sold 2 years ago, your sales allotment this year is equal to 1/3 of your existing stocks plus 30% of what you produced this year. If you didn't produce enough this year - at least 75% of what you sold 2 years ago - it appears that you don't get a sales allotment at all no matter how much stock you have. So not only do you need a minimum of 500 barrels to start a business, if you ever decide to quit the business you're not allowed to sell your remaining stock to the market - once you stop production you're no longer allowed to sell even though you presumably still have 2/3 of your stock left. (You can sell it to other producers, though.)
Now for the exceptions.
There's a weird restriction on how much you are allowed to have acquired this year to qualify for the 75% minimum, and this is one of those places where words are being used... strangely. The most straightforward way to read the restriction is that no more than 20% of the finished Port that you're using to qualify for the 75% minimum may have been purchased as finished Port. The very last section of the document throws this into doubt, however, because it talks about how the age of the purchased Port affects the amount of capacity it contributes to the sales allotment. If the phrase "wines that only attribute 20% capacity" is being used in the context of the last section, then what it's saying is that the Port you purchased to reach the 75% minimum must be less than 3 years old. In that context there's no restriction on how much purchased Port you may use to qualify for the 75% minimum, and that context seems to be the most reasonable.
The next exception comes into play if you over or underproduced this year. The basic formula assumes that a) this year you produced at least 75% of what you sold 2 years ago, and b) this year you produced between 75% and 125% of what you sold last year. A) is required or you don't get a sales allotment this year. B) has modifications that come into play if your production falls outside the 75%-125% range.
If this year's production exceeds last year's sales by more than 125%, your sales allotment is increased slightly. You still get to sell the basic law of thirds amount (1/3 of stocks + 30% of this year's production) but in addition you are allowed to sell an amount equal to 15% of the volume by which this year's production exceeds 125% of last year's sales. You get to sell 30% of this year's production until it reaches 125% of last year's sales, beyond which you get to sell 15% of this year's production.
On the other end of the scale, if this year's production fails to reach 75% of last year's sales, your sales allotment is reduced. Instead of getting the normal law of thirds amount (1/3 of stocks + 30% of this year's production) the percentage of the second part is reduced. If, for example, you only produced 50% of last year's sales (2/3 of the 75% low end of the range) then your sales allotment would only be 1/3 of stocks + 20% (2/3 of the normal 30%) of this year's production. That ratio adjusts depending on your actual production this year - the less you produce compared to last year's sales, the less you're allowed to sell this year expressed as a percentage of this year's production.
Now we get to the last section of the law, the one that's confusing. This rather confusing section appears to grant additional sales allotment based on stocks of finished Port that you purchase. The amount of sales allotment you gain by purchasing finished Port varies based on the age of the wine purchased – the older the wine, the more it increases your sales allotment.
Wine that is less than 3 years old increases your sales allotment by 20% of the volume purchased.
Wine that is between 3 and 4 years old increases your sales allotment by 40% of the volume purchased.
Wine that is between 4 and 5 years old increases your sales allotment by 60% of the volume purchased.
Wine that is between 5 and 6 years old increases your sales allotment by 80% of the volume purchased.
Wine that is over 6 years old increases your sales allotment by 100% of the volume purchased.
The last clause is the confusing one, but it appears to mean that at least half of the increase in sales allotment provided by this section must come from Ports that contribute no more than 40% of their volume to the formula. That is to say, at least half of the sales allotment increase must come from the purchase of wines that are no more than 4 years old.
So basically, you can increase your sales capacity for this year by buying stock from others, but at least half of the increase in your sales allotment must come from Ports that are no more than 4 years old.
Whew!