Basically, the idea is that the better armored a creature already is, the less effective it is to pile armor onto their scaly/leathery/chitonous hide.
So the equation should look something like this I'm sure there is an easier way to express it, but I am still sleepy
): B + [A/40 * (40-B)]
Where B = Base Prot, and A = Armor Prot.
Thus in your example, what you get is: 10 + [18/40 * (40-10)]
Again, there has to be an easier way to write that, but it's escaping me atm.
Basically you gain an amount of your Armor based on the ratio of Base/40. So as in the example with 10 Base, you will gain 75% of Armor, and if your base was 20, you would gain 50%.
Check your math! If that equation is working right for you, the answer is 23.5 (probably displaying as 24, but then you have to factor in Head Prot as well, which impacts Total Prot on a 4:1 ratio against Body Prot).