I've been wondering about how different doctor's prescribe progesterone differently too. My doctor said that my progesterone levels were within the normal range with my mc, but that what is needed to sustain a pregnancy can be different in everyone. My doctor said he would start me on progesterone once I got a + HPT, which seems like that could be too late.
So, I really don't have an answer for you, just wanted to let you know that my doctor also said that he would prescribe the progesterone after a positive test.
Good luck!