[BruceZ]

You can use the Excel function =TINV(1-p,N) where p is the confidence. So for 90% confidence after 20 sessions, you would use TINV(0.9,20) = 1.72.

Sorry, make that TINV(0.1,20) = 1.72.

I just noticed that you have 14K hands, which is 233 "sessions" of 60 hands. You shouldn't need to use the t-test with this many sessions, as any inaccuracy in your SD should be small enough to not affect your confidence interval. If you have results of each hand, then you can even use the simplified formula for SD that you used in school, since you can group your results into sessions of equal length. It will give the same result.