hi I thought id chime in on my observations and agree with many of the that percent however flawed does allow you to not get into too much trouble with either short drives and mid level number of cycles and not getting too low or too high SOC for long periods.
In My 4 years of having a MS with only a 90K pack ( thank goodness it is a version 3, that's another story). I have had a couple of times when BMS and my charging were definitely adjusting range numbers/ capacity.
one time I had to leave the car at 15 percent for a day till I could get around to charging it ( id did go into sleep) and when I did that I got 83 Percent charge ( which translated to a range change ) from where I has set it at 80%. one other time just before trip I set it for 100% and I came back and it was charged to 103% ?( another range adjustment) .
don't worry I quickly used it up so not to stay at high or low SOCS for too long. another time on distance was "hot footing" it and lost a ton of miles ( this was when SC were farther apart) . that convinced me to go to percentage, not miles.
The only way to know with any more certainly for those " detailed" types is what the posters have said about measureing pack average voltages, discharge cycles ( from when you got the car , or you will be off) and BMS information but that is probably pretty obsessive as you cannot realy control all the variables to optimize things.
To follow battery university's information which I find to be spot on for my phone as well as my car, id like to know what 75% SOC really " is" and what is 20% real "is" relative to the real capacity of all the pack is after BMS has gotten to a regular " adjustment" . so is 80% charging is really 75% of real SOC? so then you could match maximum charge cycles with out limiting Max capacity as the BMS adjusts?
then I go and drive a long distance and use super chargers and mess everything up ? See I can be obsessive too!
. I think from what I hear is don't go down to <5 % where it turns to red (unless you have to - yes I have seen it do that) , don't over super charge (that's hard cause for me its free!), don't charge a lot of cycles, but don't go full charge to discharge more than one in a while. eat your vegetables, lose some weight so the car doesn't have to carry it long term.
don't carry passengers they add to capacity reduction ( family will be not be happy) and move to a place where the climate is always 60 degrees F ( has to be on the cool but not cold side) and doesn't vary more than 10 degrees ( even San Diego doesn't count, but Ireland might ). anything less will not optimize resale.
all things in moderation. including moderation ( that why we have a 0-60 number, personally I like what it does 30-90 Mph)
(moderator note: added spaces to improve readability. no other changes made)