Does your HCG come in liquid? If it's, say 5000 iu in 1 mL, then you would add 9 mL of water and the contents to a 10 mL vial. Now each mL would be 500 iu HCG and each 1/2 mL or 0.5 cc or 50 units is equal to the 250 iu. You want.
You could probably do it with 4 mL of water too and then only have to inject 25 units. If it's powder you just add one cc of water to reconstitute it then add as above.