[binions]

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How can you guys do these calculations in the small time we have to act? Is it merely practice practice practice or we all need to have a good mathematics background?

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There's actually some symmetry to it:

For 20% equity facing a pot sized bet, villain needs to fold more than:

3SPR-1/3SPR+9

For 25%:

2SPR-1/2SPR+7

For 33%:

SPR-1/SPR+5

For 40%

(.5*SPR)-1/(.5*SPR)+4

For 50%:

You need zero fold equity to break even.

When you face a 2/3 pot bet, you simply add a little less to the denominator (ie at 33% its SPR+4 instead of SPR+5).

I have a feeling this will become almost as easy to remember and apply at the table as using the rule of 4.

After all, 3SPR (+9) at 20%, 2SPR (+7) at 20%, 1SPR (+5) at 33% and 0.5SPR (+4) at 40% is a clear pattern. Every time you jump the a categories, you lose 1 SPR and +2 off the demominator, except when you jump from 33 to 40, you lose half SPR and +1 off the denominator. But its the same ratio. If SPR = a +2 loss, then half SPR = +1 loss.