As a general "rule of thumb," I've found if you take any scope's objective lens diameter (in this case, 42mm) and add 7mm, you're going to be pretty close to the outside diameter of the objective bell. This doesn't apply to all scopes, as tube thickness varies a bit, but it applies to many scopes. Even when taking into account variances in tube thickness from mfg to mfg, you'll usually be pretty close.
1" = 25.4mm, so to convert mm to inches, divide by 25.4.
So...42mm / 25.4 = 1.654"
7mm / 25.4 = .276"
1.654 + .276 = 1.93"