A/C Charge by Myself

I was wondering about charging My AC system by myself. I have a vacuum pump I use for brake bleeding and gauges that will allow me to add R134. I have a 69 Barracuda with a Classic Auto Air unit that had to come out for paint. It is all back in now and ready to charge (yes I replaced the drier and all O-rings). Why couldn't I just vacuum the system down as much as possible and put R134 in myself. It isn't so much the expense; I just hate to drop my car off and go back to pick it up later. the few places I have called basically said just drop it off and we'll call you when it is done. I was thinking on putting in about half the charge with the AC compressor off, and then finishing the charge with the motor and compressor running. Thoughts?