Interesting about the difference in numbers, I have the "add on" digital tension readout - and with my bobbin set around 180 with the towa gauge, my top tension is set around 700 - so way, way different than mentioned above. And with different threads, I'll go between 500 and 800.
it's very possible that my "add on" unit has smaller increments, since I'm going between 300 units, and not 40-70 units. I don't normally see a change unless I go about 20 units. If I went down to 300, I'd have big loops on the bottom!