Nice. I've done some calculations as well to give me a somewhat accurate ballpark figure, but I would love to put an actual meter on my large kiln. It's firing times can very quite a bit depending on the density of the load, since it holds so much work.
(answer comes after the quote) <- Mr Stuart, this is for you
We have a small simple 3 phase electricity meter placed only for the kiln. Really easy to do (if you are an electrician) and eliminates the need for guessing.
We always write down the start and stop number from the meter and the controller programme number to our fifing log. Thanks to this, we know exactly what is going on.
I used to write down the liters of the load too but it made very little difference so I stopped (unless we fire stuff for someone else).
If we test new firing schedules, I usulay make a time-laps video of the controller read out, so we know what was actually happening. (webcam taking a picture once a minute, with a timestamp)
If you are going to buy a controller, get a one that gives you more flexibility (like Bentrup TC88). You never know, what you are going to fire one day. It's nice to have start delay, 9 ramps per program etc...