It depends also on your luck,

gen1 could have a potential between 80 and 120, because the genesis virus has a potential of 100

gen 5 or 6 can have a potential over 200 but I saw a virus at this gen with only 50.

Mutate only the best viruses to achieve best results. So you could end up with something like:

1 gen1 -> 2 gen2 -> 1 gen3 -> 3 gen4