I was thinking of doing it differently from this game as well. Basically, the first 6 heats would do each game, taking the number of missions and then halving them. Then, there would be 2 semi-finals to wittle down the number further, each semi-final having half of each heat's levels.
So, Codename 47, Contracts, Blood Money, and Season One all have roughly a dozen levels each, in their respective heats they would be down to 6 levels, and then split into 3 for each semi. Likewise for Silent Assassin, something like 10 levels since both games have something like 20 each.