When you can alternate quickly between low G and open G with no change in embouchure, and they are both in tune, then you have found the correct overall length of the instrument. THEN your tuner will tell you what the instrument is pitched at.
NOTE that I said the "overall length", not (just) barrel length - because the mouthpiece enters the equation as well. How well was MP length (and inner design) standardized back in 1888? Probably not very. So If you find a barrel you like that is long, consider trimming a MP shorter. Make a wooden mandrel on the lathe, to slip the MP over. Then cut shorten the tenon of the MP so the end is closer to the cork. Then cut back the same amount from the body just above the tenon. (These cuts are done easily and safely with a parting tool.)
Overall, go slightly short to allow for some tuning.