Grub screw hole size for threading!

[gingea1pom]

Here we go then.

I am planning to drill the holes for the grub screws in my Sierra uprights.

I vaguely remember from a fitter machinist / bench fitting lesson 18 years ago that there is a formula for working out the size of hole required when tapping a thread.

I am planning on using these (M8 Grub screws).

Therefore (Oh cleaver people) what size hole should I be drilling.

Thank you in advance.

Next If I bring my uprights to Stoneleigh could someone bring a M8 Tap to tap the thread in my new holes.

Standing by.

Cheers Ginge