It takes two readings, to tell us the range regardless of meter type. A single reading from an incident meter tells us nothing more than a single reading of a spot meter, they both only measure single points, not a range. The only difference is that with a reflective meter we have to judge the reflectivity offset, say 1-Stop for Bill's hand.
Neither meter knows anything about the films range unless we tell it, and only a select few meters will take that input and spit out range info; most of us have do the math to find the SBR with either type of meter.
See post 4. My suggestion there is an adaptation of a technique called duplexing. (See Dunn & Wakefield's Exposure Manual) Duplexing was specifically designed for finding the best exposure compromise for slide film.
BTZS uses a variation of this duplexing technique with either incident meters or spot meters, at the users discretion, to accomplish what Adams did with with his Zone System and spot metering.