give one more piece of info and it is easy
I'm playing a game with a from the top advantage of about 1,2%.
I'm going to flat bet this game.
What is the win payout or the variance?
Flat betting the initial Kelly fraction to the original bank gives a 13.5% ROR. If this is close enough you can find the bet size by using either the win payout: f=ev/win payout, or a close approximation using variance. If it is a bj game with a rule change that gives you a bigger edge then it may also increase or decrease variance. In most cases it is a bigger premium on a natural or some other hand, which increases variance. A bonus that increases the frequency of payouts will decrease variance while one that increases the payout of infrequently occuring hands will increase variance.
If we just use an approximation of 1.33 for variance, then your fraction would be: .012/1.33=.00902. For a $1000 bank this would be bets of $9. You probably would not go much beyond your 15% criterion if you bet $10, and would speed up the game vs making multi-color bets.
I assume that you were saying a 1.2% edge, although you said 1,2%.