Not enough miles, what is the threshold to get the extra payment option?

Hi KLM experts,

I have been into collecting as many miles as possible through several ways and booked some great direct flights AMS - LAS this summer.

For one the trips, I did not have enough miles, but was close enough that it let me book with less miles (30k instead of 40k) with an extra payment of 100 euros.

My question is, when does it let you do this? I assume you can't be more then 10k off? I could'nt really find it anywhere, so I hope you guys can clear this up. Thanks in advance!