What is the correct value for "certainty" in RSA key pair generation?
Certainty of $x$ bits means that the probability that something (in this case $p$ being prime) not being true is smaller than $2^{-x}$. This is the same probability as guessing a random $x$-bit value correctly on the first try, hence the name.

How to select $x$? We want the probability of $p$ (and $q$) not being prime to be small enough that a failure probability in this point is not larger than other ways the system could be broken - like guessing a symmetric key, factoring the modulus etc.

So here a correspondence table of symmetric and asymmetric key sizes should help. Pick the same prime certainty as you would pick an symmetric key size accompanying your public key usage.