Fundal height question

Elephant in the hospital
Just wonder because it seems to vary by doctor's and web page. When baby drops do you measure from your pelvic bone (if had 2 doctors do this) to top of sack or 2in above pubic bone (if had 1 doctor do this) trying to see if I measured my baby right (I am not a doctor I just try to keep track and if been pretty accurate to my doctor's measuring)