At the risk of being eviscerated - my reasons for 80 verses 100 or 50 as starting point when paying to fill a tank
1 - 100% gets you 2200psi, 80% gets you 3100psi here
2 - I typically have 15 minutes or less deco with first stop rarely deeper than 30'
3 - I dive open ocean and use 15' as my last stop
4 - I easily transfill my "80%" tank from a hp 120 of 36% - I transfill until it's a high 40 number then will pay to have it refilled - I can get 4-6 dives from one "paid" fill
5 - I love the precision of tech diving, hitting my gas changes and planned theoretical run times, perfect stops - but I'm in no hurry to get my head above the water line and neither are my friends - once deco is complete, I'll hang there on back gas with you till your done, who cares!!
6 - I dive in Cozumel, the water is warm, your group is close by - ehhh- relax, finish your deco...
7 - It's illogical to pay for a fill that you really do not need - these are 15 minute or less deco dives that are done all the time.
I understand standard gases when you're talking 30 minute or longer hang times but as long as your gases are close to each other for shorter hang times, ehhh....