I don't understand this concept. You should be using your PTO, not cashing it out!
Once I realized banking PTO right before quitting meant it would get hit by income tax, SS tax, and Medicare tax, I realized it was much better to use the time off.
Personally, I sell(some) of it back because my company pays time and a half for PTO CashIn(the same deal exists for vacation time, but I have my limits)(I get floating holiday time, sick time, and vacation time, all in separate buckets, with different restrictions and rules on each. I'm only speaking of selling back my sick time, which I could use on vacations if I wished). I can do a bit more with the going rate of a day of work(and would love more overtime to soak up the $'s) than I can with the time off, for now. The calculation changes as I get more towards the FU money stash/FI and older end of the spectrum though, and I also have a pre-FIRE break planned which changes things up for my specific situation, but that's my thoughts.
It's another 3-5% of pay I can get per year. A day at work instead of taking the time off is basically $500 in lost potential income. And if we do a quick calculation that we all love, $500 saved means $20/year essentially in perpetuity, or $1000 in value 10 years from now.(or ~4k in value 30 years from now). For my specific situation, that day of working is basically equivalent to a week of not working during my employment break, which I'd rather think of than a day off, today.
(My employer takes out some percent of PTO cashin as taxes automatically, and they also have settings you can hit to make it automatically fill up 401k at a different percent than regular paychecks. Your PTO cash in value will depend on state laws and the employer themselves.)