Calculate the required paint weight (using recommended gsm or oz/sqft), then tape a weight in the middle of the control surface area to simulate this weight and size your ballast accordingly. This should get you pretty close. You can always take out further weight after painting by drilling into the side of the lead weight in the future, or you can add some nut plates now so you can add bolts and washers if necessary. I added two nut plates on each side, but did not need them in the end. Don't forget to account for the elevator trim system weight. The electric requires a significant amount of extra ballast. You're better off having a little extra now, then drilling to remove some in the future once you've finalized your paint. The most important thing is that if you fly painted or unpainted, you have the surfaces perfectly balanced for that flight condition so you don't get flutter. Another upside to painting before the test flight; you only have to balance once.
Tom.