The idea is to slowly build up to 12 reps at a specified weight and then increase the weight when you can do the 12 reps with ease.
Unfortunately, I can't hold more than 70 lbs (my arms give out and I can't get the straps over my shoulders where they're supposed to be) so I can do 12 sets of 12 reps at 70 lbs and not even feel it.
It sounds like you're into serious weight lifting, so I doubt a bowflex is the right piece of equipment for you.
If you can do 4 set of 20 with a hard 5 count squeeze at the top, you'll have some serious abs going.
Some people over do their abs in the misunderstanding that doing ab work gets rid of belly bulge and love handles. It don't work like that. You don't lose fat locally. Your body make use it up at different rates, but just working a flabbly middle will mean that you have really toned muscles under your belly bulge.
Which is where I'm at right now... About an inch of fat in front over slabs of abs. More on the sides and enough on the back of my wasit line to give me "potato butt". It all needs to go. I'll never be under 200 bls again, but I'd at least like to get back into the 220's.