In the ideal situation, you would want to force purge with co2 from a co2 tank right after tossing the sugar in. Just a couple of simple purges at like 5psi is all you need to do right after priming. After that you'd want to let the co2 build up in the keg naturally, and not touch the release valve. If you don't have a co2 tank to force-purge in the beginning, that don't purge at all. You want to let that pressure build up in the keg and not release it for proper carbonation.