WizzM, here is a way to measure for ring height that is posted on another forum.
"...there is a systematic way of calculating ring height that doesn't require more than a simple elementary school ruler. You measure 3 things:
1. the height of the front of your base/rail from the barrel
2. the height of your rings from the top of the rail to centerline
3. the outside diameter of the bell of the scope (not the size of the objective lens)
Here is the equation:
[rail height + ring height] - [bell diameter x 0.5]
If the value is positive, then your bell will clear the barrel. Optimally, you want that bell to be about 1/4" or less over the barrel. This will give you a tight cheek weld and provide for more precise trajectory. Be sure to account for Butler Creek caps if you intend on using them.
If you are using a 40mm objective, then you can use the lowest rings and bases available, as the bell will certainly clear the barrel with no problem.
40mm or less - low rings all the time
50mm obj - low rings on < 5 contour
Medium rings > #5 and up (strait barrel get med high)
56mm obj Medium on < 5 contour (sometimes med high)
Medium high on sendero contour
High on >7 contour
50mm objective - lows will work all the time
56mm objective - lows < sendero or #5 contour Sometimes lows will fit number 7 contour barrels, but it depends on how long the shank is after the smith installed it.
Highs for sure anything larger than #7"