To be honest the machine shop i originally took it to to get a quote told me that it`d be fine to touch the starter ring and it wont cause any problems as its pressed onto the fly good and proper so drilling into it a touch wont really disturb it.
I never did see how deep my mates work that i eventually got to do the work went. Pretty sure i told him all the info i`d been told and let him get on with it. He would have just kept going until the bolt could screw far enough in. I didnt have the need for washers so. My mate gave me some antirattle washers but working in a race gearbox firm they all use them but on a road car something like that isnt needed lol.
The measurement on ETKA for the bolts are M8x16