I believe what you're looking for is in section 5 of API 650.
Recall that the pad thickness used in the calculation is the minimum of:
opening diameter OR the actual pad thickness
Example: a 250 mm diameter opening with a top pad of 300 mm and a bottom pad of 150 mm. Count top pad thickness at 250 (minimum of opening vs actual) and bottom pad thickness at 150 (minimum of opening and actual).
Converting energy to motion for more than half a century
