Working out Calories for Turbo Trainer with Motoactv?

geebusuk

Just got myself a turbo trainer to get some cycling done regardless of weather (and I'm assigned to puppy-sitting duties this weekend, not sure if a five month old whippet can manage the >40mph descents on my usual half hour route!)

Trainer: Elite Mag Speed Alu
Computer: Motoactv with motorola HRM and speed/cadence
Bike: Charge Scourer

First focus is calorie burn as I'm nearing the end of losing 4 1/2 stone, so trying to be careful about calories in vs calories out.
(Though, of course fitness is still important - as well as general fitness, hoping to maybe do some tri's, as well as doing occasional XC events etc - may do some road events too, just for a bit of different fitness.)

On the road, the motoactv seems to provide a fairly reasonable figure for calorie burn.
However, not sure how accurate it is/will be on the turbo trainer with variable resistance. At a low resistance it seems a bit too low, but at a high, I reckon I'm putting in more effort that would burn that number of calories on the road.
If I set it to 'stationary bike' it doesn't pick up speed/cadence data, so I've been using normal cycling, but set to indoor so it doesn't try and get a GPS lock.

I've tried to find some data for the resistance offered (it has five levels), which I presume would let me work out what was going on and thus the accuracy, but can't find anything.