Fixing linear rail - DIY practice

Linear profile rails have torque recommendations for the fasteners used -from what I've seen, something akin to what a grade 10.9 may be torqued up to. Also, there are recommendations on the thickness of the material being bolted into - usually typically the depth being equal to the depth of the rail being used. The depth of the supporting material obviously adds stiffness, but is also required for thread engagement and a factor of safety on stripping at the recommended torque tightening values. I've got the impression folks drill and tap as low as 3mm thickness on square hollow section and drill and tap to fasten linear profile rail to it, which by the numbers I've briefly looked at is nowhere near enough for grade 10.9 type torques. Do people here do any numbers / calc's for the permissible torque values for fasteners in such scenarios?

I used 50x50x3mm box section on which to mount my profile rails but first I looked up the dimensions of standard nuts. This gave a good guide to the depth of thread required for any size of bolt. So inside my box I glued a piece of 6mm thick flat bar using Gorilla Glue, and a good job it made too. This gave me a thread depth of 9mm into steel, albeit mild steel.