I have some doubts about what is the correct torque for holding the Almen strip in the Almen block before doing the Almen test. I haven´t found anything related to this issue in the standards, and I have seen "slight" differences in the test results when the Almen strip is fixed with different tightened level.

When you say slight how slight? Odds of getting the exact same reading from strip to strip are not that good regardless of torque. Generally the screws are hardened and they will snap if you apply to much.