IIRC, the overflow (actually a reservoir, not quite the same thing, but whatever) is supposed to be filled to the "Full" mark when the engine is at operating temp, and between Full and Low at when cool.
MK3 TSRM On-Line
You may feel free to overfill the reservoir if you're burping the system or unsure if you've got a leak and want a little security. After a hard drive the reservoir will purge the excess and level out right about where it needs to be. This is the best way I've found to burp the system with little effort although, you'll be nervous about the green puddle under the car for a little while.
