I have to agree with TAUTO depending on how maney horses you are intending to back.
If it is 3 or less then just place the same amount on each horse unless you are constantly backing odds on favs i find this the best way to go....if you get a winner at less than $3 it can more than be made up by the winner at $6.

I use a bank percentage bet of 5% devided by three this is calculated at the begining of each day.