Fitting is a matter of trust, communication and experience. Trust that the fitter is not intent on selling you a bike they have (this is not to say that retail shops don't do good fitting, just that there can be a conflict of interest). Communication between you and the fitter is essential. Do they listen yo you, invite your feedback? The fitter's experience in the range of riding and body type you have is also important. A fitter that only does triathletes could be insensitive of the needs of the tourist.

Getting fitted is somewhat like going to the Dr. Your back pain will have one solution to an Orthopedic and another solution to a "shrink".

Having said all that if you question the bike builder's fitting numbers (and comparing them to your current ride filtered with the explanation given by the builder/fitter) then do get a second opinion. Andy.