Some nice additions you got there!
It seems to me that you only use target numbers that are multiples of 5. In this case, a roll of 1d20 < TN/5 + 1 approximates 1d100 < TN, with a difference of exactly 1 percentage point in success probability.
I often houserule away d100s this way when they're not otherwise needed as is the case with F.O.R.G.E., but YMMV.