There is a formula (Klein Kelly criterion-see wiki) which describes the % of portfolio one should dedicate to a bet.
The problem is-it involves assigning a probability to a trade success, which is almost always an impossible task.
Example: on a trade with 60% success one should invest 20% of the portfolio; trade with 70% success, 40% of portfolio, etc, etc.
Mind you, these %% are "real" numbers, not imaginary ones.
Also, NO MARGIN.
EDIT:
Formula...Fraction of portfolio to invest (in decimals)=[2X(chance of trade success in decimals)]-1
Therefore, 0% to invest when 50:50 (or lower) and 100% to invest when 100% chance of success.
Everything else-in between.
Actually, at 50:50
Klein Kelly says "invest nothing", right - but if the odds are under 50-50, the Klein criterion suggest the opposite bet should be taken, with an amount again determined by the formula you quoted (it will come out as a negative fraction).
Example: on a bet with 40:60 odds (that's less than 0.5), the criterion suggests 2 * (0.40) - 1 = 0.8 - 1 = -0.2,
that is, invest 20% on the OPPOSITE bet (if anything like that exists).