I used to be on Testogel which - I suppose - is kinda the same as Androgel? Idk, it's gel in a tiny bag anyway.

Anywho, I was on half dose for the first three weeks which meant half the sachet per day. I used the remaining half the next day and didn't have problems. Some say you should discard the remaining half but I closed it as best I could and put it in a fridge.
Of course you can't be 100% sure you'll get the exact half a dose this way but that's how the doctor instructed me so it can't be all that wrong. *shrug*