[quote=So Boerd]
For ease of arithmetic, how about 2 bullis per soldier when raiding farmland to a max of a quarter of the population?
[/quote]

That's actually pretty clever. You eliminate the minimum troop number, but still keep true to only being able to get half the pop. number's wealth.

That said, it would be odd, since if a fief can only be raided once, all anyone would have to do to protect their own fiefs would be to 'pillage' with one soldier.

I think your idea is a good, but there should also be a minimum of 100 soldiers.

EDIT: Perhaps whatever is looted is simply reduced from that annum's tax, instead of the pop. not being able to pay taxes that year?