Strange that they quote it in kg (mass), rather than N (force). Not a big deal to x10 though !

I've always used the following as a guide:
Wood 10 N
Alum 50 N
Steel 75 N

These were quoted by a forum user a long time ago, and I've seen 'similar' values quoted from a company that make force sensing equipment and had kitted out a milling machine. Of course it depends on F&S too, so just a start point.

But remember these become moments when applied to the gantry so are a force at the tool cutting edge x the distance to the gantry bearings. Yes this will try and bend the gantry, but it will apply a much bigger twisting force and this will dominate the total tool deflection (in terms of how the gantry reacts).

This is also just the mean static force. There will also be vibration on top of this which will add a bit more to the total force applied.