No problem here - just saw a need (probably misplaced) to smooth things over with you. Like I said, it's a cue - something I think is obvious that you might not.
I think that "backspacing" is an un-official standard - which is often supplanted by "offset." Catch is, I'm not sure which way is positive and which way is negative - but I'm inclined to think a negative offset would be going outwards, and positive would be going inwards (but don't hold me to that.) Either way, 20m/m is a little over 3/4" - figuring from a 3.5" centreline, that would come out to either 2.75" or 4.25" BS - depending on which way they're going. At least, that's my read on the situation. I'd ask for an explanation in "rich dummy" terms - because I tend to simplify things myself, and I remember information in its simplest form. Again, that's just me.
(Oh - I don't have enough hair left to part anyhow. Don't worry - no offence here, either.)
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