Soil warming cable / propagator

Im considering building a large bench propagator for the greenhouse and was looking at the parasene soil warming cable with thermostat.  There doesn't seem to be much difference in price between long and short cables so I was thinking of making a 6'x18" box. Im wondering about the cable length, heat output and running costs.

Having done all the calcs I was about to buy a 12m cable when I found this link http://www.twowests.co.uk/twowestslibrary/soilwarmingcables.aspx which suggest 6m will be ok but I would be at the top of it's capacity. Given the running costs are double for the longer cable it caused pause for thought. I'm looking to germinate earlier and grow green leafy things later in the season. 

Does anyone have first hand experience or sage advice ?

That was my initial reason for over-sizing the cable length, my back of fag packet calcs showed a gap of 3" between cable coils on a 12m run I wouldn't want to overheat the cable and damage it.

I would want to waste time with something which wont reach/keep a decent temperature - I've monitored one of the cheap electric internal propogators whilst it's been in the greenhouse and it's struggled to keep +3 degrees above ambient at night 

If it is of any help I use a thermostat heating and cooling controller, with an under floor heat matt.
It has worked very well over the last few years. you can use the cooling on it to turn on a fan if the temperature
goes above your set-point. you can get them on fleebay. my friend uses them on his egg incubator.

http://www.ebay.co.uk/itm/Temperature-Controller-Thermostat-for-Reptile-Snake-Lizard-Incubator-Aquarium-/200866250814?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item2ec48fc03e