Unless I have misread the above from Gorephoto, you are only using the FBH when the engine is running, The reason you have "never looked back" is because you will have not placed a direct drain on the battery before the engine has been started.
But,- you are not having the advantage of a pre-warmed car when you start the engine. If that is the case, you may as well have saved all your trouble and just use the basic 5 degrees or below temperature auto start, or the override method of manual start, dependent on which model FBH you have.
To improve on the above, The general Idea is to start the FBH about 15- 20 minutes before you need to start your journey. In that time, the FBH warms up the engine coolant, and after about 12 minutes the coolant is warm enough to be able to start the ATC, with the manual settings suitably set, to warm up the inside of the car. This then melts all the ice on the windows all round, and you drive away in a nice cosy car, while your neighbours are still scraping the ice away.
What you must cater for is a suitably timed switch off of the FBH so that it does not stay "switched on" until the next time you need your car.The favourite trick, developed on this Forum, is to incorporate a spare heated seat switch into the circuitry which has a 15 minute timer built in to shut off the FBH. Your car may be warm enough after the first 15 minute period. if not just run it again for another 15 minutes.
There are masses of information on on this Forum for the FBH and flat battery prevention. Again, apologies if I have read it wrong
PS When I started my reply, none of the others had been entered.