Originally Posted By: Kuato
Just fill until you can reach it with your pinky finger. If it's overfilled, put a drip pan under it, grab a beer, and wait for it to stop dripping
(or siphon some out)
^ This. Stick your finger in there, you will have to bend your finger to fit it in the hole, when you feel the tip of your finger is wetted by the lubricant level, stop filling.
Next time you think of it, go to a Veterinary Supply vendor, buy some honkin' syringes (you don't need the needles, although they can come in handy, especially if you need to glue something tricky) and some tubing to fit the end, and you can remove lubricants that are over-filled in pretty much any area of the vehicle, reasonably quickly.
You can get syringes designed for the purpose at automotive stores or woodworking supply shops as well, but they are a) more expensive and b) not as well made (tend to leak past the plunger).