I agree with all of the above. I have "messed around" with doses from pens, but I am capable of calculating the doses accurately and keep a spreadsheet plus Shotsy app record of every shot from every pen over the last year+ that I've been on MJ.
I also now share pens with my husband, but use them as vials with separate syringes, not needle tips that can potentially backflow. He started after me with his own prescription, but as he had less weight to lose, he was on maintenance long before me. As he is on 2.5mg (not 2.5ml!) for maintenance, I just order the level above the one I need. Eg I order a 12.5mg if I require 10mg for myself.
I also don't use the official dosage steps, but move up little by little, often 1mg or less increase every few weeks, max dose ever was 10mg. It has worked for me. Minimal side effects, 100lb weight loss achieved and now at target BMI 21.
I'm not a poster girl for following the rules absolutely, but have a legitimate prescription from a legitimate source, was a qualifying BMI 38 at the start, and have never increased doses faster than the schedule, only slower.