It does depend on what you're working with. When you're working with a mix, you calculate using the peptide with the highest amount to figure out the bac water ratios. So, if you have a 10mg/5mg mix, you would only use the 10mg for your calculations.
For BPC157/TB500, most are 5mg/5mg or 10mg/10mg. If you have a 5mg blend and your target dose is 250mcg (0.25mg), you can use a peptide calculator like Prime Peptides version. Select 0.25mg Dose, 5mg Peptide Strength, then choose the amount of bac water. It'll show you how much to draw.
If you're using an insulin syringe, you'll want to match it to a major mark. 2.0mL matches 10 units on an insulin syringe. So, 10 units will give you 250mcg, 20 units 500mcg, etc. Injection pens are more precise, so you could put in 1.0mL and make it a 1:1 ratio.