I've actually made a rediculously long post on my findings when experimenting with the zombie spawn system and I've come to the conclusion that adding just two settings, which could be very easy to implement, will greatly improve your control over how you want to ramp up and/or down. And that is something like Halfway Population Multiplier and Halfway Population Day that function pretty much in the same way as Peak Population Multiplier and Peak Population Day, but you can set those somewhere in between the start and Peak Day.
Add something a little harder to implement like an Ignore Respawn Unseen Hours Hours (yeah how you define these things might need some naming work) with a high threshold and you'll be able to make sure that a cell keeps firing respawns during this period and afterwards still have a high Respawn Unseen Hours (like 3 months or something) so you can actually clear the zombies from cells later in the game and not end up with a ghost town in the beginning.
I go in a lot more detail in my post but it still has to be approved by a mod (as does this post, so when you read this I guess my other post will also be approved and you can find it in the General Discussion. Fair warning: it's a daunting read that by virtue of all the parameters I've tried to understand and describe also can be rather confusing.)
Edit: Yay approved, here it is