Flask recharge mechanics

Does the game round flask charge fractions up or down?

If I gain 1 charge on crit, and have a 20% increased charges gained belt,
does the game count it as 1,20 and then rolls it down to 1,00 ?

Do I need at least 51% increased charges gained to get 2 flask charges per crit
(in this example every crit gains 1 flask charge) ?

Flask charges are gained as partial amounts. You can have 0.2 of a charge, for eg. It rounds down when displaying, but it's still there. So in your example, you would gain 1.2 charges every time. This would look like you were gaining 1 charge every time, until the 5th time when it would look like you gained 2.