the key to jackhammers is clean smooth crickets you should be able to do as many as you want with out tiering yourself and make sure that your stabed arm is stable not wobeling around
"There's one life to live so live it the best you can, The world could use one less man" -NAS
your crickets are sexy as hell, just keep practicing your jacks. practicing more circkets wont help anymore. just keep doing jacks fatigue your wrist and wait for the rewards in a couple of days.
I'm not an expert at this but i think your problem is that you don't lean on the side you're doing the circles.You are standing too paralel to the ground.