Learn this technique and you’ll never need to wonder again:
The tricky part with leather is that it’s really analog. Variations in thickness are just part of the process. For each sample of leather I do an independent test, there’s just too much variability to have a “one size fits all” setting for all leathers.
As for masking, sometimes I do, sometimes not. Depends on the surface of the leather and what I intend to do with it. Leather is a massive rabbit hole and I’m barely looking down into it at this point. @morganstanfield no doubt has some good posts on leather working, if you search the forum for “leather settings” and “leather tips” I bet you’ll find a lot of info.
As for “messing up”, I mean, experimentation is part of what makes it great! Lean into the discovery process, and I bet you’ll figure stuff out really quickly.