i know its been asked loads and loads of times but want an accurate useable thickness

i know they are 139.1mm as standard

how much is taken off in each skim normally,or does it all depend on damage(how much is needed)

ive heard people saying they know people on 138mm flat but im not confident with this

as others say 138.6mm min

been offered 1 at 138.7mm

but dont want to spend a few quid on 1 that might be on its last skim

cheers