Without actually seeing the software, sometimes a BMS will set the range a little bit low if you did not do a full charge. A full charge lets it see and re calculate the true min-max if you consistently fail to bring it up to max so it can see it, it will drift based on inputs from past driving histories, etc and wheedle it's self down. Eventually you may get error messages about balancing like the Energica will do to you. sometimes if it is severe enough it will want you to do a full cycle, ie take it down to zero, then back to 100 percent so the bms can totally reset itself and say THERE, NOW I got good numbers, and then go from there.
Also know that the lower your percentage goes down, the faster your range will go down as well, it needs more amps to go the same speed because the battery voltage is lower so less power. The extra current draw will further suppress voltage and further make it look like the battery is depleting even more.
since so many things can affect range, temperature, battery age, condition, terrain, traffic etc etc. and range can vary from bike to bike, brand to brand etc, it's best, especially if this is a route you normally take. do one round trip, charge it paying attention to how many kw you put in. repeat this a few times keeping tabs on the number, then you have an average. ok it takes 7 kw to go 70 km. so my range is 10km / kw as example. from there you can look at your battery and see about how much range you have left IF you drive the way you normally have before.
If you see your percentage seems to be going down a bit more than expected, then adjust your perceived range accordingly.
If you need to know at different charge states, do the charge thing at those percentages. run it charge only to 90 percent a few times, then 80 percent a few times, and see what that does for range and what it reports. That is about the best way you are going to get an accurate feedback for YOUR bike. You are getting real numbers from real application of your riding experience. Not estimates or just voo doo math.
One main take away, your bms MUST know a hard top and bottom for your battery pack togive you what it estimates is range.. as this drifts from constant up and down without completing a full charge, what it reports is going to drift as well.
Aaron