So if it's a Ruger factory, rail then use then
10 inch pounds with a dab of LOCTITE.
The manual doesn't give you a number, but I have one on my Volquartsen/Ruger and that has worked for me.
They rail has been on for 4 years now.
Follow the advice in post #2 and #7.
And be aware that there is no force trying to loosen your rail except the expansion and contraction of heat and cold.
The lateral recoil forces against the screw is tiny.
Smooth