I prefer to clamp at about 40 to 50% erection and increase the tightness slowly as well. I do not need any more stimulation to increase erection; I just kegel more blood in then turn the screw on my clamp a little more and repeat this until fully erect. Once I am fully engorged, I push the clamp down towards my pubis as I kegel more blood in; it makes it far easier to get the blood past a tight clamp.
I think it is important to allow the tissues to ‘relax’ into the expansion, so I allow my erection to go down a little before I kegel it back to full strength again. If the clamp is very tight, this slight loss of erection occurs because of the tissues relaxing rather than blood flowing out (although a certain amount of blood flow out is going to occur).