Bike sizing for the longer leg... Cannondale synapse?

the hub theory works for some but at the end of the day its just a ballpark estimate that doesn't take into account individuals needs (ie flexibility). Knee over spindle also only has limited theory and to many is flawed. It is tricky when you are not average height. I'm 194 with 100cm legs so I struggled getting something to fit as the bigger you get, the longer the top tube which wasn't actually what I needed.

It would sound to me like a 61 might be too big if you are stretched out now, you can go up to a 14cm stem on a road bike, and many find a longer stem nicer as you get more relaxed handling. Only way of knowing is to sit on one and thats your problem, 61 cms cannondales are not widely stocked. No one can give you a firm answer only view I'm afraid.

I really wouldn't get hung up on your view of the hub position. I only remember that fitting gem from rigid MTBs years ago, on a road bike with drop bars and the multiple hand positions they offer then I think it's not really applicable as you'll move your hands around a lot as you ride altering your view, the angle of your back etc.

Looking at the C'dale site there's 2cm difference in the horizontal top tube measurement between the sizes. Doesn't give the stem length for each one, so hard to guess which would be best.

However, with legs that long would it be fair to say you've got a short (it's all relative ) torso for your height, if that's the case then the slightly shorter frame makes sense.

Have they got a demo bike you could hire and go for a proper ride?

Or, try something like the Bike Fast Fit on an iPhone/iPad in the shop. You'd need to stick the bike on a turbo but if the shop want to make the sale and you take a packet of biscuits then they might go for it.

Are you anywhere near Ludlow? Epic Cycles are known for being good at bike fitting and they usually have a big stock of Cannondales.

It's a long time since I first heard that rule of thumb and not thought about that in years. After reading your post first thing I checked my bike this morning, hub exactly in line with the bars with my hands resting on the hoods. I'm slightly taller than you but my legs are slightly shorter. Not on a Cannondale but riding XL / 61 cm frame.

If you felt stretched on the 58 then maybe that's the right size for now. Any improvement in flexibility won't come overnight and if you're not comfortable then on the bike initially then you're less likely to enjoy it.

I've never heard that hub rule of thumb, but I'm a similar size 194cm and long legged too. A CAAD10 60cm fits me perfectly, once I went to narrower bars. Cannondale scale up the bars a lot more than other brands. I had a 46cm c2c on mine. So check the bar width if you are not broad shouldered.
Wide bars effectively lengthen the top tube, but cause shoulder pain instead. I really like my CAAD10, great bikes

61cm will be the better option for you however cannondales do tend to be longer than other brands such as trek, if your flexibility is an issue get a shorter stem and dont alter the drop right away.

too many people drop there bars down because its what the pros do, but half the folk are no where near flexible enough for this.

for the reach, you dont want your arms locked out, make sure there is a little bend.