Converting the 28/38/48 chainrings on the commuter/touring bike to 28/42/bashguard. Shimano Alivio 104/64 BCD crank and chainrings.

I put the 42T meant-for-outer chainring in the middle position with the bashguard on the outer position and the gap between the 42T and BG looked too close. Re-mounted the 28T inner and the gap to the 42T was too big. I realize now that the outer chainring curves outward from the bolts to the teeth.

Anyone know off-hand the thickness of spacers needed between the crank spider and the 42T chainring to put the teeth at the same relative chainline as a true-middle chainring would produce?