Canyon's geometry specifications for their Aeroad and Ultimate include Reach, Stack, Reach+ and Stack+. While the former 2 agree with all other bicycle manufacturers measurement methodology, Reach+ is defined by Canyon to be the horizontal distance from the bottom bracket to the centre of the handlebar tops (where your stem clamps to your bars). Stack+ is defined by Canyon to be the vertical height between the bottom bracket and the top of the handlebar tops. 

Now I have been looking at both the Aeroad and the Ultinate in a size L. Stack and reach are listed as 580mm and 401mm respectively. Canyon goes on to state that Stack+ and Reach+ are respectively 656mm and 490mm. Canyon claims their integrated stem/handlebar is effectively a 110mm stem. O.K. there's an angle involved, but wouldn't one expect Reach+ to be measuring more than 76mm greater than Reach?

It's correct because the steerer angles back 73 degrees, so you lose 3cm horizontally for every 10cm of height.

Then I may be able to calculate reach+ on my current and prior bicycle that had 120mm stems.

What about stack+? Any formula for that as well?

thanks

Stack or stack+ is just a vertical measure. But reach is goofy because it relates to stuff that is constantly modified by the angled steerer when you compare two bikes, raise your stem, etc.

Everyone should have a drywall square Measure the distance from the floor to the bb shell center and then measure from the floor to the handlebar tops.

Do they reference the handlebars because they have integrated stem/handlebars?

I can see this as an easy means of measuring Stack+, however I don't know of a similar easy way to measure Reach+.

Then compare virtual top tube dimensions. Look, there are always fudge factors in how far you're going to reach for a handlebar, anyway--stem stack, stem angle, stem spacers and compression cone height, handlebar reach, width, and sweep, the brand and model of your control levers, and where you prefer to sit on the saddle.