If there were a lot of applicants then maybe the Secretary problem would be applicable, but considering how few there are that can decrypt it i doubt there's an optimal split to be found theoretically. It probably comes down to how greedy or stubborn Stefan is, since theoretically he would profit even if he only kept 1%, but I doubt he would agree to those terms. 50% sounds reasonable to me.
https://en.m.wikipedia.org/wiki/Secretary_problem