Ok so 250 Test means every ml out of that vial is 250mg. So 0.5 ml is 125mg.
So divide the mg you want by the the mg of the vial.
Example 1
You want to inject 200mg from your 250 mg vial.
200 divided by 250 equals 0.8
So you draw 0.8 ml
Example 2
You want to inject 75mg from your 250mg vial.
75 divided by 250 equals 0.3
So you draw 0.3 ml