6MP is dosed by body wt -- usually 1.5mg/kg of body weight so your dosage of 75 mg means you shouldn't weigh more than about 50kg (110 pounds). If you weigh more than that maybe you need an increase dose of 6MP to get you back in therapeutic range.
I think it starts that way and then they do blood tests as you progress. I’m about
190 lbs and I couldn’t tolerate more than 25 mgs per day because of the way my body metabolizes 6-mp.