Depends what crank you're intending to use. XT FC M730 (and DX, LX, deore equivalents) would be 122.5-127.5mm. Slightly later low profile cranks like LX FC M560 would be 113-118mm. Later still micro drive cranks (22/32/42 chainrings) would be 107-113mm. The shorter options give 47.5mm chainline...