Your question as i understand it is "can a battery meter meant for SLA give an accurate reading on Lithium batteries?"

My answer is, probably not. But in reality, battery meters are fairly inaccurate, period.

Lifepo4 batteries aren't too far from SLA batteries in terms of voltage. Lead Acid '36V' rated batteries are actually 39V when fully charged. LiFePO4 36V batteries are fairly close -- a full charge might be 40 volts (mostly depends on the design of the charger!).

Likewise, when you are NOT running a current, your LIFEPO4 should be about 36 volts when it gets to low voltage cutoff (that is to say if you keep using them at that point you will substantially reduce the life of your batteries). The same is true for your lead acid batteries.

If you want your batteries to last as long as possible, try to keep them at 'all bars' or 'all but one bar'. If that doesn't work for you, don't worry about it. USE your batteries rather than treating them like they should be kept in a display case. (they will degrade over time, whether you use them or not).

It's not at all bad for your batteries to be discharged to low voltage cut-off ONCE. Check how many miles you get (use your normal riding style as far as how fast you go, how hilly is the terrain you choose, and how much you pedal). Then you'll have a better idea what percentage of your battery life you use up to go 15 miles.