I have done some manual clamping before and I have done it wrong, but still I remember, that if I have done some warm ups slowly increasing the pressure, to the point when I clamp., then the girth was much bigger, (at least for me) from 5,07 I had 5,24 which is visible and especially felt in hands. So if this is the point which certainly is, Maybe clamping will be the right way to go.
suckerpunch