that's how it works, the exact formula is kept secret by wetsuit alone
but there are a few factors related to how bonus % on each level is computed:
decrease (-): amount of free roll claim (total claimed)
increase (+): by playing multiply btc (total wager amount) and/or buying lottery tickets
by my observation, there are 20 levels and we start with level 1 as a base reward (currently 32)
then for each level up we get additional amount of base reward (bonus level = multiplier)

in your case you are on bonus level 4, then your free roll is (3x32)+32 = 128
you activated 1000% FREE BTC bonus, 32 x 10 = 320
so you can claim for 128+320 = 448 satoshi each free roll at this moment

edit: made a small correction on the (+) sign