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.
Like patt0 mentioned out, if we don't limit a max bet stake, everyone can just bet his all bank in the final moments of the competition, and with a lucky bet they will claim first prize, and that seems unfair to someone that was able to make small bets throughout the entire competition and showed overall profits.
There is another problem with that system, that is the risk for someone to go all in and make big bets, when having a small initial bankroll. If someone starts the competition with 1 BTC, and another one with 10.000 DOGE, it will be a lot easier for the one with 10k Doge to go allin if things are going wrong, and that does not seem fair as well.
Also, in your example you chosed to make a big bet at low odds and a small bet at high odds. That is exactly the right thing to do, since the chances on winning at high odds are lower. The only problem is that you chosed to bet 5% in both of them, so that good decision was not reflected on your bets at all.
You would not get negative ROI% if your 5% was the 0.1 and not the 0.01, that way your bet would not get limited.
So to solve your problem in the next competition, see what is your normal betting stake, and list your initial bank so you can bet that stake as 5% of your bank, and everything will be ok.
Please remember that you don't need to fund your address with the bank you declare, so you are free to state any bank you want, allowing you to use you normal betting stake as if it was 5% of your bank, even if that represents a bigger percentage of your real betting bank.