Flange Holl Marking Formula

This will depend on various factors. here is one template:

TEMPLATES FOR DRILLING FLANGES

This is not done by formula, but by looking up in a table, as in post 1. There are additional tables for other pressure classes of flanges.

In these what do they mean by 1) US standards - 125,150lb-is it force or pounds/sq ins

2)In DIN std -PN,DN 3)In ANSI-class 150,300 etc 4)In BS- Type A,B,C,D etc ?.

For an answer, refer to those relevant documents. You may have to buy these books.