Question : Surgeons Flask and Pathfinder's Master Surgeon

Could anyone clarify how surgeons flasks work?

First Question:
Is the 20% chance to gain a charge rolled when the cast/attack is rolled as a crit, or is it rolled on hit?
Example: Multi strike attack rolls a crit, hits 3 times. Is 20% chance rolled 3 times, once for each hit? Or is it rolled once, just for the skill rolling a crit?
If it is the former, is there a limit on charges gained, like the legacy version? (limited to one charge per cast/attack rolling as a crit)

Second Question: How is pathfinder's Master Surgeon rolled? Is it 10% rolled on hit, or rolled on crit?
Is there a limit like the old surgeon mods?

Third Question: Are pathfinder's Master Surgeon and the Flask Surgeon Mod independent, or additive?
Example : When I crit, will I roll 10+20=30% to gain one flask charge, or will I roll 10% for one flask charge, and a separate roll of 20% for one flask charge, giving me a small chance to gain two flask charges?



Background
Legacy Surgeons : Recharge a flask charge on critical strike
100% chance, but limited to one flask charge per cast/attack
Example : Multistrike rolls a crit, hits 3 times, but only recharges one charge, instead of 3. Spell echo rolls a crit, hits 2 times, but only recharges one charge, instead of 2.

Thanks!