Hi Arthur49, glad to hear you are enjoying yourself.
I originally flattened my battery to the point of low voltage cut off based solely on information gleaned from reading posts on this site. My decision to do so was not based on any data or knowledge on my part. It certainly didn't seem to do any harm, however, I would be inclined to do as D8veh says, as he is clearly far more knowledgeable about these matters than me. Certainly, if you do flatten the battery, charge it at once and don't leave it sitting around. Nowadays, I fully recharge if I do 5 or more miles, and I have never flattened the battery completely since my first ride. I also found that range appeared to increase a bit after about 300 miles, but that could well be the effects of my legs becoming more efficient.
I assume you have the 16 A/hr battery. Mine was allegedly 17.5, but I'm sure they are all the same. You will learn about power consumption with experience. On mine, when I have 2 lights showing in a static no load state, there is approximately a quarter of a tank left or less. I am of the opinion that the long range is the best bit about my Eagle. Mostly my range is limited by a sore bum more than battery range. However, if I can anticipate an opportunist top up en route, then I can do long rides on full power, including some off road stuff up on Beachy Head without any worries about running out of electrons.
My battery has shown some signs of lowering capacity, but it still has plenty enough for a 40+ mile day out. It's almost 2 years old and has done around 2500 miles. I'm sure I will get at least a couple more years out of it.
Glad you like the bike, I still love mine, and I always get a little bit excited every time I set off for a whizz.