Calculating scope to rifle body clearance
The way you calculate the clearance is by halving the overall OUTSIDE diameter of your rifle scopes objective lens and then deducting half the rifle scopes body tube diameter.

For scopes with a 1 inch diameter body tube this means deducting 12.5mm.

Note we say OUTSIDE diameter - this means NOT the lens diameter, you need to measure the whole outer casing diameter.

The result gives you the minimum "saddle" dimension required - the measure from base of mount to bottom of the ring = the 18mm saddle.

The amount the saddle dimension is greater than the figure you have calculated is the amount of clearance your scopes objective will be clear of your rifles body.  The saddle on these mounts is 18mm

Example -

1. A scope has an outer case measurement of 58
2. 58 divided by 2 = 29mm.
3. 1 inch tube scope so deduct 12.5mm from 30mm = 16.5mm
4. So the mount must have a saddle measure (base of mount to bottom of the ring) greater than 16.5mm or else it will ground out on the rifle body.
5. So in this case with 18mm it will fit nicely. 

Calculating your rifle scopes clearance in this way does assume your base is not raised above or below your rifles barrel. Many weaver bases are raised to some degree which may well affect the sizing consideration.

Note some rifles with high or adjustable cheek pieces may require higher scope mounts or risers so that your eye is in line with the rifle scope and your head is kept in an upright comfortable position.