Here's a decent little write-up on voltage drop testing the starter motor. Note the total limits for drop and the component limits. Ford solenoids, or "starter relays" as they are sometimes called, are notorious for having high drops. Fortunately, they are the easiest component to change out as you don't have to jack the car up nor bend over the fender much.
https://www.aa1car.com/library/voltage_drop_testing.htm