well, if you change the decay mod to be something like (1+numHeroUnits)/(factorial(numHeroUnits+1)), and then modify the other numbers a little bit, you'll end up getting another hero really fast in the early game, but every hero that you get drastically reduces the chance to get the next. On the other hand, should you loose a hero unit, it significantly increases your chance of gaining another adventurer.
I was originally thinking 1/(numHeroUnits+1), but if you make it a factorial you can leave other numbers much higher, allowing for much easier getting of new hero's, and drastically reducing the chance of ending up with 10 hero's by turn 200. Just fudge the numbers so that you start off with about a 7% spawn chance.
This would end up being 7% chance until you get your first, then a 7% chance for your second, a 3.5% chance for your third, 1.17% chance of a fourth, etc. If you want to have a higher chance of getting more, you could change the top to (1+numHeroUnits^2). This would give you a 7%*x chance of getting a new adventurer, with the first being x=1, second x=1, third, x= 5/6, fourth x = 5/12, etc.
-Colin