Wall Mechanics

[IdesOfMach]

Alright, so I've been writing a spreadsheet to use instead of the in-game Battle Simulator. I have the bulk of it figured out, I'm working on the last three modifiers: wall, morale, and luck. Morale and Luck seem fairly straightforward as percentages, but the wall is a hassle, since it combines a base increase to defense, as well as a percentage increase.

In essence, a Level 0 wall has 20 base defense, and then increases by 50 per level. It also has a 0% defense bonus, which increases by 5% per level, with some exceptions, I'm assuming due to rounding error. Namely, 3, 4, 8, 13, 18, and 21 are each 1% below the expected value.

How the simulator takes these values into account while calculating the effect of a Ram stumps me. If anyone has any insight, please share