If the boats on the hard, I understand his thought process. If you want to go to the top of the mast and make the measurements, and "do all the leg work" you could save alot of money over having the rigger do it, IF it's in the water.
Be careful going up the mast and use safety precautions, and have spotters.
Above all be safe, and figure that facet into his estimate, as he is not going to put his life on the line for you, and you should not put yourself at risk, if you are unsure about any of this. Above all else, be safe !
P.S. Most riggers I know work out of a facility that has a lift and working area for boats so transporting the mast any further then necessary is lost on me.
Best of luck