You should use 600 ml of developer, or find the plastic clip (or jerry-rig a clip or retainer) and make sure the reel is all the way at the bottom.

If I were still using the tank, for black and white processing, I had been thinking I would use the proper amount of stock to make up 500 ml of developer - and then add even more water to bring it to 600 ml, then I'd adjust my times to compensate for the slightly reduced activity of a bit more dilute developer. I would rather develop in a solution that was a little too weak and risk slightly thin negatives, than have only part of my negative be properly developed.