The usual approach is to add enough bac water so the final injection volume matches what you prefer to draw. Concentration has some practical upper limits though they are not strictly known. For GLP-1s I generally aim for 30 mg per ml.
I handle mixing fine but still get unsure about how many ml of bac water to add for each peptide vial. A 500 mg NAD vial gets 3 ml, a 10 mg Reta vial gets 1 ml, yet 40 mg Reta or Tirzep vials throw me off. More bac water means more dilution, so less bac water should give higher concentration. What ml amount works for a 40 mg vial?