@Saracen - in the longer term if you had some means of measuring how many watt hours you used/consume for your different rides then you will over time get to know how much power you are using for your different riding scenarios.
Once you know this and the capacity of your battery, you'll become quite good at working out your range.
I don't have a watt meter on my bike but back in the bike shed I have a home built rig that;
(a) measures how many watt hours I have to put back into the battery to get back to the fully charged state (having started with a fully charged battery of course) and then log the miles done, type of terrain etc. to obtain the watt hours per mile figure mentioned by Neal in post #6
(b) I can also discharge the battery into a dummy load to measure the capacity in watt hours.
These measurements aren't overly accurate but over time have been enough for me to be confident in what range I'll get for any particular ride - so no range anxiety for me lol.