Adding 2 cups of 6% bleach in 500 gallons would raise the FC by 15.4 ppm. Are you trying to decontaminate your spa or did you want the FC to be that high? With your numbers, the bleach would have the pH rise to around 7.7. Don't forget that high FC levels above 10 ppm can lead to a falsely high pH reading as the high chlorine level bleaches out one of the two indicators in the pH test. The pH will come down some as the FC drops. When the FC is closer to 5 ppm or so, check the pH.
If you are using the Dichlor-then-bleach method, then using 50 ppm Borates is recommended (after your TA is low enough to have reasonably stable pH) as this will help to stabilize the pH further, reducing the fluctuation in pH when bleach is added and then consumed.
Also, note that Clorox 6% regular unscented bleach has the least amount of excess lye in it, but if you are using an off-brand Ultra bleach (or an off-brand regular bleach, though that is really weak at 3% or less) then they usually have a lot more excess lye in them so generally aren't as good to use unless you don't mind adding acid to compensate for the rise in pH.
Finally, you will probably have a stable pH target in the 7.5 to 7.7 range -- unlikely to be much below 7.5 so don't try to fight it.
