Thats virtually the same as the one i used to ferment my Larger kit, it seems to have done the job ok for me as far as i can tell. If it is the same idea as mine it has suckers fixed to it, so that you can stick it to the inside of the brew bucket.
I took a wile to get the temperature spot on to start with, i kept turning the heat up and down till i got the temp right.
I think the best bet might be to just use the same amount of water that you will use in your brew, until you find the right temperature, then it will be spot on ready for your brew. Then there is les chance of allowing nasties in to your bucket or jar or what ever you are using.
My heater has a thin nob on the top to increase or decrease the temperature, obviously different amounts of water will require a different setting on the heater.
Good luck with it.