Hey Full.contact,
Try riding strapless first off and take note of where your feet are when you are comfortably powered, this should be the rough indication of the positioning for your straps and plugs.
Back strap is usually straight and front is either straight or slightly biased to your natural or goofy stance which ever you prefer. Don't cut into your stringer, just place inserts either side of the stringer and epoxy glue them to the stringer for added support and strength.
Most wide footstraps have allowances for either twin screw setups or single. Single is most common Screw either side of the stringer for straight and use twin screws or just use a slight heelside bias or slight diagonal bias for the front strap and go single, just dont exagerate the angle too much or it will be painful for your foot to ride opposite tack when you gybe.
The other alternative is to go with Youngy's theory and place the front strap across the board at 90 degrees to the stringer then it doesn't matter either way and you wont ruin your stringer.
Chopping or grinding your stringer will result in a broken board in the not too distant future.
Good luck and go with what feels right for you, after all, it is you that will be riding it!

Good winds,