Ananvil:
I think the perception was that those things added weight, and thus if you added more than one, mass did become infinite. If the total mass it set in the beginning, I'm still confused on how the % works.
I see. the hull that you choose is the total displacement of the ship. So if you choose a 100-ton ship, it will displace ("weigh") 100 tons. When a system requires, say, 10% of the hull, it occupies that much space. So on the 100-ton ship, a system that requires 10% takes up 10 tons. This leaves 90 tons to be allocated to other systems. If you were to add another system that requires 5% of the hull, that system would require 5 tons of the total 100 tons. After accounting for those two systems, you would have 85 tons left to allocate (100-10-5).
So the initial tonnage is the empty hull that you will fill with stuff.
You should check out the sample ships, I think things will make more sense.
And the mass does not become infinite - if the % values were added instead of a percentage of the empty hull, you would get a finite number. The 2 values would sum in a (quickly converging) infinite series, but it would still result in a finite number. This math is not what is intended, however.
Don't worry too much, I will either revise the document to be more clear, or write a new set of rules.