There is no universal way to accurately determine clamping force based on torque applied to a fastener. There are only rules of thumb that are no where near the accuracy of what you would like.
For mounting it is far more accurate and practical to use springs. With these you can measure (or calculate) force versus deflection. The force really isn't that critical unless:
1) You use thermal tape (final thickness varies with force)
2) You use too much goop (similar to #1, but not as severe)
3) You have excess tubing hanging off the block or tubing with tight bends close to the block (tries to "pry" the block)
4) In air cooling if you have a "tall/heavy" fan arrangement (oversize fan or tall funnel, which will also try to pry the block off the CPU)
More important for four-holers is equal pressure at each corner. Compress each spring an equal amount and you should do fine. As for final force, compress them in your hand and get a general feel. Ideally, the springs are part of a good kit and have already been selected to give the right force at a prescribed level of compression.
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