Chombi1,
For best result, I think that you need a jig...can't find anything on the web to help you.

A few weeks back I was over hassling a local frame builder and IIRC he had a template for drilling either stem or seat post from back when making bike components to make them lighter and therefore faster was the rage.

I'll try and trace it down and at least photo it for posterity. Maybe if you have a name of a maker you could locate the template that you need.

Update:
https://www.instructables.com/Make-Y...ke-components/