To get the round trip Gouy phase of the cavity to 1/3, the total round trip length should be 2.4 m.

Current mirror locations: (X,Y) =  (10.655 in, 1.1033 in)

Current total round trip length = 4*X+4*sqrt(X²+Y²) = 85.47 in = 2.17 m

Need 2.4 m, so need to add 0.23 m = 9 in

This means every mirror needs to go 9 in / 8 = 1.125 in back

8 comes from 4 mirrors and the round trip

New mirror locations: (X,Y) =  (11.779 in, 1.163 in)

New total round trip length = 4*X+4*sqrt(X²+Y²) = 94.46 in = 2.40 m

Thus, I plan on buying sixteen 1.125 in spacers for the every mirror.