I would like to propose a better system. The 5% limit is really troublesome. My bets are usually more than 5%, but it gets rounded down. So when I make a 0.01 bet on high odds and lose and a 0.1 bet on low odds and win, my ROI goes down, while it shouldn't, imo.

A better way is to take out the bankroll part and the 5% limit. (10x can stay, no problem with that).
Bet with whatever amount you want.

So, ROI = total of payouts/total of bets.


Edit: In that example, it would be (say odds for the first was 3x and the second was 1.2x)
1st case:
Bet = 0.005 Payout = 0
Bet = 0.005 Payout = -0.005
-50% ROI

This one:
Bet = 0.01 Payout = 0
Bet = 0.1 Payout = 0.12
ROI = 0.12/0.11 = 9.09%

The negative 50% kind of thing is the reason why my ROI went down from some 80% to 12%.



Edit2: Since ROI is a %, both the existing and proposed ones are independent of the bet size.