From the poolmath site:
Note: pH calculations are not exact. These numbers are only suggestive of the relative magnitude of the pH change you can expect. Small changes, +-0.4, with pH between 7.2-7.8, TA around 80-120, and Borate near zero will be approximately correct. The further you go from those ranges the less these pH changes will correspond to reality.
How inaccurate will the numbers be when you have borates at 50? I'm about to have to add some acid to lower the pH since my borate addition, and I'm assuming it will take more acid to do it, but I am unsure how much more...or is that just for the chemical calculation and the top portion of poolmath is going to be more correct?