About the initial question: A drop of 10% switched off/without doing anything is not normal for a bike (maybe for a Tesla...). It's normal that the battery level reduces maybe 1-4% after charging (so you charge until 85%, then stop charging and one hour later it shows maybe only 83%). But without charging, if you come home with 61% and just turn of the bike, it should still be 61% also 3.5 days later. Maybe 60% if you already where near 60%, but not less. Even with a new battery this is not normal. Just observe this and if it continues like this and bike was definitely turned off, ask your LBS.
About battery guides: Many are much too long but the basics rules are much shorter to say, it's not a rocket science:
1) Assume around 50% as the best point for a lithium ion battery
2) Battery stress increases with distance AND time from this point. Don't be afraid to use full capacity your battery in both directions, but only if you really need it and then go back to around 50%, the sooner the more you are away from it. Leave some reserve above 50% according to your "use case".
3) (Very) high battery temperatures are bad for it, but they appear very rarely in actual e-bikes, just forget it. Very low temperatures will slow down battery's work (charging and discharging/motor power). So in winter charge your battery when it's warm/directly after the last ride.
Comments
at 3) The cold needs time to "creep" into the battery, so cycling in winter or storing the bike some time outside is no problem. Only if stored for a longer period/all night long at -10 degree Celsius outside, of course battery will frozen in the morning and can't be charged/no electrons will enter... But storing a lithium-ion battery outside in winter doesn't hurt it (this is a common myth), also charging outside in the cold is no problem, if the battery is warm enough/not frozen.
at 2) Examples:
- You get home with 1-10% and need the bike the next day for a long tour with maximium range-> Charge the battery immideately and up to 100%
- You get home with 1-10% and know you won't use the bike for several weeks -> Charge the battery immideately but only up to around 50%
- You get home with 1-10% and aren's sure about the next tour lenthg and if you need the bike tomorrow or in three days - Charge the battery immideately and up to 70-80% (this is one of the most common/practical charging states for normal use for most of us). If you then know that you will do a long tour with maximum range in a few hours, just charge it again for half an hour for the last %.
- You get home and know you need the bike the next day, but maybe not 100% range or you can live with a few miles without motor -> Charge up to 90%, much better for the battery than 100%, but almost full range. This is the other very common daily use case.
- You get home with 20-30% -> Charge the bike soon, but it's not the end of the world if this will be the next day.
- You get home with 40-60% and aren's sure when to to use the bike next, in two days or two weeks -> Do whatever you like/relax, no real need to charge.
- You charged (maybe by accident) up to 100% but then realize that you will not use the bike for weeks -> Try to reduce the charging state at least 20-40% before that long pause.
A plug with a timer (WLAN plugs with app control are great and cheap) is useful to control the charging. Just measure roughly how much % your charger puts in the battery per hour. So you know how many minutes or hours you need to charge to get to a certain state.
Perfectionist who use the bike daily for example to get to work even charge in two periods. One coming home directly after the ride maybe up to 60 or 70%, one in the morning 1-2 hours before the ride for the last 30-40%. But in winter be aware of the cold if bike/battery is stored outside, then perhaps you can't charge in the morning and need tou charge everything on one step after coming home.
. My only means for meassuring the battery is Mission Control. 
