The confidence band available in the reference lines is for the confidence interval for the sample mean estimating the population mean. If you add a reference line for both average and confidence band, you will notice that the confidence band centers around the mean. As for all confidence intervals, the general form of a confidence interval is (statistic) +- (critical value) * (standard error) which in this case is (sample mean) +- (tcrit(n-1)) * (s/sqrt(n)) where tcrit(n-1) is the critical value from a students-t with n-1 degrees of freedom and s/sqrt(n) is the formula for the standard error of a sample mean. Of course this is different from the standard error of a sample proportion which is what you need. Tableau uses the standard highly accurate polynomial series expansion to approximate the student's-t distribution rather than a table. But, Tableau does not provide direct access to this distribution.
Are you sure you need that much accuracy in your t-distribution. By the time you get to 2000 degrees of freedom, the normal distribution is an extremely good approximation for the students-t. I have described how to approximate the normal distribution with a Table Calc here: t-test of two independent means
You could improve on the normal approximation with an approximation for the students-t.
See my recent addition to the forum post above for the formulas.
With regard to using your table, that would probably be possible as well.
But, perhaps more complicated than necessary.