"When you start pouring in the developer or when the last drop of the developer goes into the tank."

When all of the developer is poured in, and you have slamed your bubbles out. Start the timer and then start your initial constant agitation.

That will help ensure even development. Avoiding processing times less than 5 minutes will also aid. If I can't hit the 5 minute mark with my film / developer combintation, I go with a different dilution, or I use a different developer.