I have a reserve on my '98 Tomos Targa LX. I haven't ran the gas low enough i needed to use the reserve. What i do is to see if the gas is getting low on the bike is to shake it from left to right when standing between it. This gives you an idea how much gas in the bike. After a few times of this you'll be able to approximate the gas level really well. Hope this helps you.
Swarm and Destroy!