In article , Jinx
writes
Anyway John to answer your first question, The measurement refers to the
depth of the lock.
Yes

i.e. the distance from the front edge of the door to the center of the
square bit that the handle fits too.
No, it's the case depth, the measurement from the front of the lock to the
back of the case. The dimension you describe is the backset.

Current chubbs are mm sized, key posn 40mm back from the door edge is
likely to be a 67mm (2 1/2") lock, 57mm backset suggests 80mm (3"
lock). I say likely 'cos they do a 3" lock with a 40mm backset too.

You make the backset/case size mistake once, travel 100mls to middle of
nowhere to fit the lock for aged relative, then you don't forget ;-)
--
fred