Hi
I've not tested the syntax on this, just written it, if you use it please in test mode first.
Amount field in your betting trigger: IF(test_mode=1, IF(test_funds>40.00, test_funds*(perc/100),2), IF(total_funds>40.00, total_funds*(perc/100),2))
where perc is the percentage of available funds you want to bet
change test_funds and total_funds for test_balance and balance if you wish.
or, as per your second option, in the settings:
settings > betting options > allow small bets