I need some help and/or understanding, please

The math lines up with what should happen, so the drop to four doses is expected. The 4 mm PES filter absorbs around 7 units each time you filter. Multiply that by four doses and add the dead space left in the syringe and you can easily lose the equivalent of two doses. Using 2 mL of BAC water instead would get you closer to the six doses you were counting on.

A video shows how much liquid can disappear during filtering and why that happens. It might clear up the volume loss part.

You would need more details to pinpoint where all the liquid is going. A 13 mm filter can hold back 15 to 28 units. I use 3 mL and 5 mL syringes for the BAC water and filtering steps; those hold back a few units themselves and are less precise. I usually see about 10 % loss on small volumes. I push some BAC water through a new filter first, then when filtering the reconstituted solution I leave extra air in the syringe so I can force air through at the end to push out a little more liquid. I also draw at least 40 units per dose for these peptides and 50 units for other compounds to reduce the effect of filter losses.