Kelly says you should invest $x\%$ of your bankroll in a gamble:
$x = \frac{pE-1}{p-1}$
where $p$ is the probability of winning and $E$ is the expected payoff multiplier if you win (i.e. $E$ times how much you bet).
But lets say you have two options to invest in: One that has high risk and high payoff potential and another one that's absolutely safe, but only pays very little (e.g., FDIC insured bank account).
Is there a formula for this case? I was thinking to subtract the yield of the absolutely safe instrument from the expected payoff of the high risk venture and then just using the formula above.
EDIT: I found the formula: [pE-(1+r)]/[E-(1+r)] where r is the interest rate