One idea I have, that I have yet to try out myself simply because my home gym setup needs a little work to make it happen is as follows:
Attach a triceps rope to the high position of a lat tower with a long cable (x-over tower is probably enough - i.e. a multi-pulley system), put a little weight on the stack (the less the better), hold the rope over your shoulders, face away from the x-over tower and go down in the GH exerise.
(The trick is fixing a "GH device" in front of the tower...that's my problem right now). When you're at the lowest position, the small weight you're holding onto will provide some upwards pull, courtesy of the lat-tower weight stack; so you won't need to "push-off". YOu increase or decrease weight accordingly, and you're all set.
Progression comes from reducing weight after every workout or so till you don't need it any more (then you start ADDING weight). This quantifies your strength, which is better than trying to judge the force in this weeks push versus last weeks.
I am not yet strong enough to do GH raise without push-off, and like you, I never really know how-much force I use.
I agree with you that low-rep sets is right for GH raise. For me, they are too intense to do more than 5-reps (even with a push-off).
As and when I get my setup working at home, I'll post a picture (darn site easier to understand than my typing I'm sure).