Some decent advice above, but I'd like to clarify. Voltage after you stop, is a very good indicator of state of charge, at the end of the discharge. Say, from about 80% used up on down to 100% used up. The middle will be more like the same voltage for a large part of the discharge, 75% discharged may not look very much different in voltage than 60% discharged, and even 40% discharged will be pretty close to the same as 75%. So judging the exact middle, 50% is almost impossible by voltage.
Ok, so what is the bottom voltage with your pack? I can't say, because I don't know what chemistry your pack is. If it is made from the typical round 18650 cells, then the lowest possible voltage for each cell is 2.5v. If you have lithium cobalt, like RC flat pouch cells, the lowest is 3.5v.
Assuming you have 18650's, then 2.5v x 20 cells in series gives you 50v as the absolute lowest possible empty voltage. But since you have 20 cell groups in a series, your pack will have its bms shut off discharge when any one of the 20 groups hits 2.5v, or perhaps a bit higher like 2.6v. So don't be too surprised if your pack shuts off when it gets to around 53v.
At 66v, your cells are at an average of around 3.3v, which like we all said, is about the middle. But somewhere in the middle between 70 and 40% discharged.
Based on just some experience, what you should get riding pretty fast, how far you went, you are in the middle on that ride. You should expect 20 miles per ride, but maybe less if you really haul ass. You have enough watt hours for 30 and 40 mile rides, but much slower and pedaling medium hard.
All you can do is run the battery a few more times about that 10 mile distance, and recharge full after each ride. Wait a half hour after the charger finishes, then unplug the charger and plug back in. If it starts charging then, let it charge and repeat, till it won't start recharging. This is balancing the pack, and getting every cell as full as possible. Do this once in a while as long as you have the battery.
Once you are sure that the battery fills well each charge, time to ride it a long ways, and get it empty or nearly empty. It does not really matter how well your watt meter is calibrated, what you need to know is what the total ah you have is when you use it all. Then after that, you know that with this battery, using that wattmeter, you expect to have x amount. That is your real capacity, even if the measurement is not perfect. Divide that in half, and now you know where 50% is, in ah. Simple math also now gets you any other %, in ah.
Now, when you reach 80% discharged, you can either stop, or at least slow down a lot to take it easier on your cells. They won't like a lot of amps discharge in the last 20% left. Assuming you get about 18 ah of real world capacity from that pack, measured with your watt meter, you need to slow down and pedal more when you get to about 16 ah used up.
When you really haul ass the whole ride, don't be too amazed if your total capacity is only 16 ah. Those capacity tests they do that rate your cells at 20 ah are done at a rate that would be similar if you rode 15 mph. At 30 mph or more, you will get less capacity.. Where did it go? the cells got all hot. If your battery gets really hot every ride, SLOW DOWN. It should get quite warm, but if you cant hold it in your hand, its getting killed.
