Testing the pen itself for accuracy should take care of any inconsistencies. Larger injection volumes can also raise the chance of injection site reactions. I've always heard the reverse: less BAC water gives more precise dosing. I normally use 2 ml BAC for most things except one peptide where I reconstitute with 2 ml, draw it into a 3 ml syringe, split it into two cartridges at 1 ml each with a fresh needle, then top each cartridge off with another 2 ml of BAC.
I'm moving over to a pen and came across the idea of using twice the usual water when reconstituting to cut down on dosing errors and leftover waste in the cartridge, which seems sensible. With 30 mg of peptide in a 3 ml vial that I plan to turn into 6 ml total liquid and split across two 3 ml pen cartridges after filtering, how should I handle it? My initial plan was to add 3 ml water to the vial, pull 1.5 ml of that mixture into a syringe and filter it into the first cartridge, then pull the rest and add the same amount of fresh water before filtering into the second cartridge. Any better approach that keeps the peptide-to-water ratio the same in both?