How much is enough?

I've always wondered; how do we have a meaningful debate on how much to spend on defence?

For example, if I were to say, can we stop buying additional F-15SGs, some would naturally say something like "You can't put a price tag on peace".

But we all know that this is not sustainable in the long run.

We have to be able to say, based on our projections, X amount of weaponry will be more than sufficient.

It doesn't help that much of what we need to know, to decide on that X amount, is off-limits due to Operations Security (OPSEC).