As Allan said, several % of the main battery will be used on a long ride, especially when the RE charge level is already low but you require some more assistance.
It is the Specialized LBS that will do your RE updates. Of my 4 RE's only one has been updated. Indeed the rest of my REs throw error codes or switch the e-bike off at very low RE charge %. I do not care very much about it as I avoid riding at the RE charge level below 10%, when the assistance is greatly reduced.
I usually set the "Discharge Range Extender First" for the week-days when I mostly use my Vado SL for short local rides. For a long ride, I discharge both batteries equally (the system indeed takes care that the Wh left in both batteries are equal). When the main battery level has dropped down to some 10% (or even only 20%), it is the time for me to insert a fresh RE.
I could ride for some 147 km on a certain day swapping the REs. The 70 miles of that ride involved pretty fast gravel group ride. Assistance of 80/80% was required to keep with the group (taking my ailing legs into account).
I also need to mention I have recently participated in two gravel/dirt group rides at a racing speed. Vado SL would barely do the job (100% Turbo would be a tad too slow!) so I had to ride the full power Vado 5.0 there.
