Lyophilization (freeze-drying) removes water under vacuum so that a peptide cake can be stored for months. Reconstitution puts a known volume of solvent back. Bacteriostatic water (0.9% benzyl alcohol in sterile water) is the usual laboratory diluent because the alcohol slows bacterial growth in a multi-use vial.
The arithmetic is linear. Concentration (mg/mL) equals milligrams in the vial divided by millilitres of water added. Dose volume equals desired micrograms divided by concentration in mcg/mL. An insulin syringe marks 100 units per millilitre, so units = mL × 100.
A worked example
A 5 mg vial of BPC-157 reconstituted with 2 mL of water is 2.5 mg/mL, or 2,500 mcg/mL. A 250 mcg research aliquot is 0.10 mL, which is 10 units on a U-100 syringe.
That is the entire calculator. Vought ships it so you can see the numbers move, not so a machine can dose anything living.
Technique, not folklore
Aim solvent down the glass wall, not onto the cake. Swirl, do not shake — you are resuspending a protein-like polymer, not making a cocktail. Let foam collapse. Aliquot if the working stock will be opened often. Label the concentration, the solvent, and the date. Unlabelled vials are how laboratories lose weeks.