2003 KX 65 race sag

I'm a relative "newbie" when adjusting suspensions. I own a '01 CR125 and have read to set your measurement for sag at 100mm. That is the target mark correct? Well, in an instance of adjusting the sag for a '03 KX65, what measurement do I shoot for when it comes to son's bike? Is it still 100mm? Seems kinda much, or is it a percentage of 100mm because its considered a mini? My son is 51 inches tall (4ft. 3in.) and weighs around 55 lbs. The bike seems kinda tall and stiff suspension for him also. Any hellp would be greatly appreciated.

Thanks,
Chad