Pressing: Buy in amount?

Uth
I start at $20/$20 and buy-in for $500 (25x ante bet).

I press $5 only if I win 4x bet. If I lose, drop to $20/20.
If I get to $40/40 and win, then take profits and drop back to $20/20.
(I don't play Trips unless full house pays 9 units)

I was thinking of pressing $5 on every win.
In the long run, it'll be the same 2.x% house edge.
But pressing more leads to higher variance so I need a bigger bankroll.

How much bigger buy in to compensate for the increased variance?

And why that amount?
Is there a formula?

I tried it with same buy in amount.
I pressed after each win, even if it was 1 unit. (Ie: dealer didn't qualify so ante want paid)

I was up both sessions but 1 session was because I got quads at my pressed bet.

Hmm.. $15/15 and pressing $5 is 33% increase.
But at $5/5, pressing $5 is 100% increase.

If I lose $20/20 with Max bet, my session bankroll can take the hit.
If $5/5, I press to $10/10 with max bet and lose then that's a Large hit to my session bankroll.

So start with $10/10 (with matching buy in) as the minimum for my pressing strategy?