Please find below a list of some 3d STL vendors I use and what scale I personally use to scale them to 15mm. You can find them on https://www.myminifactory.com/ and https://www.patreon.com/home

Forest Dragon: 150% (160% sometimes?)
KMFP: 180%
Mr M ORK: 45%
WakesEmp: 220%?
RaiderOfCendre: 220%??