I've been using the TFP system for 5 years, and subscribed to PoolMath 2 years ago after the old website calculator started to lose some functionality. PoolMath does all the math as advertised, and the log tracking and periodic maintenance reminders are great for a pool owner that wants to minimize daily maintenance.
My only gripe has been PoolMath seems to have lost the ability to change your 'Target Values' and keep those custom values each time you revisit. For example, I prefer to have my FC levels at the top end of the recommended range so I don't have to top up my chlorine levels very day. I can change the value from 6 to 8 as I'm entering the test results to get my value, but when I close the results, that 'preferred level' resets to 6 (middle of the range) every time. PoolMath seems to do this for all values! FC,Ph, TA, CYA, Etc… I thought when I had first subscribed to PoolMath two years ago, my preferences were being retained, but I may be imagining it, all season this year so far PoolMath resets my preferred target values every time. It's a minor inconvenience, but having to remember to change values each time mes it difficult to maintain a consistent level which is what I'm after.