For mm to inches, I double the mm, then add 20% (or just a bit, if I’m just getting a rough idea), then move the decimal over one spot. For me, that one’s easy.
Inches to mm I have a harder time with, but I dunno why. Should be just divide by 4 and subtract a bit, but I always have to think that one through more than once.
Stupid non-conforming base 10/base 12 systems… 😒
Ryan/// ~sigh~ I blew up another bowl. Moke told me "I made the inside bigger than the outside".