Simple mathematical progression, for me, also a 5'8" person

:
- 31" - OK for up to 1 hour
- 32" - OK for up to 2 hours
- 33" - OK for up to 3 hours
- 34" - OK for up to 4 hours
- 35" - OK for up to 5 hours
- 36" - OK for up to 6 hours
- 37" - OK for up to 7 hours
- 38" - OK for up to 8 hours (domestic first class seat)
- Int'l Business - Over 8 hours