My bike is the "Masi Uno Drop track"
it has a 73.5 degree head-angle.
my handlebar is the DEDA CRONONERO TRI Bullhorn
actually i'm running the "Deda Zero Uno 2014 Stem (120mm length)" with 6* degree riser, i have flipped it down to -6* degree.
i have my stem as lower as possible, i got all the spacers above the stem, i like the handling feel it gave me..
when i'm on the bike in a comfortable position and looking straight down i can see that my hub is 2cm back from the center of my handlebar
i'm about buying the same stem with a different length.
should i go with the 100 or 110mm ?? (actually i got the 120mm and is too long)
So long as your seat height and setback are set correctly, then I would say you probably need shorten the stem by 20mm. One way to try this out before buying a new stem is get the stem used from a LBS or on eBay. For the price of a new stem you can buy a 100 and 110mm. The stem length is roughly correct when in the drops your: 1) shoulders are not hunched forward and 2) with your forearm parallel to the floor your upper arm to forearm angle is roughly 90 degrees.
Are you actually trying to determine the proper stem length by looking the handlebar obscures the front hub? That has nothing to do with proper fit - the question you should be asking yourself is how does the reach to the handlebar feel, not what it looks like. Not to mention that the reference point of the handlebar obscuring the hub is meant for a drop handlebar with the hands on hoods being further out in font of the tops than they are on a bullhorn.