Since you are withdrawing to the 3.5 units, I'd put that much air in the syringe and inject that into the HCG vial so you don't create a vacumn.

You won't spill it, you can even hold the vial upside down when you pull the needle out of the vial, it won't leak as long as you are not pushing on the needle's plunger.

No powder came out.

Inject it where you like.

Go fast if you like after waiting for your skin the dry...play with it and figure out what you like.

I'd just leave it at 350 per doc's instructions.