When does the HCG "doubling factor" go away in pregnancy?

I am Currently 8 weeks. On my 7 week ultra sound the doctor detected a strong audible heartbeat with a previous HCG of 47902 three days prior.. However my beta that i gave the afternoon of the ultrasound only went up 45% to 73,300. He told me that once he sees a heartbeat the HCG numbers doesnt matter to him anymore. Does this sound right?