Depends on your blend. When doing a mixed blend you calculate the peptide with the largest amount for the bac water calculations, so for example a 10mg/5mg blend you only use the 10mg for calculations.
Most BPC157/TB500 currently are 5mg/5mg blends or 10mg/10mg blends . Lets say you have the 5mg blend and the dosage you are targeting for the protocol is 250mcg(0.25mg), using a peptide calculator like Prime Peptides version you would select the 0.25mg Dose, 5mg Peptide Strength and then choose the amount of bac water you would want to use which will then show the amount the syringe needed.
If you are using a standard insulin syringe, to make it easier you will want to match it up to a major mark, so 2.0mL will match to 10 units on a insulin syringe. So ever 10 units will give 250mcg, 20 units 500mcg, etc. Injection pens can make it more precise so you could put in 1.0mL and make it 1 to 1 with 250mcg=250mcg with it.