To repeat others comments above, i would suggest you go to a dealer if in doubt. Personally, I have gone from a 56 Cervelo S2 to a 56 Evo (i think on paper the top tube measured horizontal on the S2 is about 5mm longer, although there is also a 0.5 degree difference on the seat tube angle).

My build isn't finished but am dialling it into a bespoke Bikefit sizing model i had done a while back (and used for all of my bikes). I am running a straight (non-layback) seatpost on the Evo and 120mm stem and should be spot on.....

Me: 5"10, inseam 84 cm, saddle height about the same 74.5 cm as you.
On a 54 frame (56 is definitely too long), with no additional spacers, but tall Evo cone spacer (about 25 mm), 100 mm -6 degree Syntace stem I have about 10 cm drop from saddle to top of handlebar.

folks here seem to have a strong tendency to ride very short and low. at 5'10", 54cm is on the short side and 10cm is quite a lot. im just saying that i doubt a 56 is definitely wrong for you.

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Having longer legs I need a shorter frame.

5'10" and 74.5cm saddle height doesn't really jump out to me unless with 190mm plus cranks.

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Thank you, that is the kind of drop evidence I am looking for.
My 745 is actually low. I have it intentionally sent a little low to correct a bad habit of toe down pedaling. If I lower the saddle height a little I do better at getting my heel down.

Ok, whatever. I'm not going to argue my fit over the internet. I had a 56 Jamis before and I like it better with a smaller Cannondale.

Using stats from USA military anthro data the predicted 95% confidence interval for inseam for a 5'10 male is 84.7cm +- 5cm. So your inseam is pretty much right down the middle of the population - neither long nor short. Note that I'm not arguing about your fit - your saddle height is what I would expect it to be and a 54 is a sensible size - I'm just making the point that the inseam ratio is not a constant across the height spectrum as some fit systems would have you believe - it scales allometrically.

timzcat - a 58 Evo would only mean a 1cm shorter stem required than the R3. Which could possibly be made up with bar reach if you don't like the look of a short stem. You actually have a few different sizing options. If you give more position data (saddle setback, reach to bars, bar model, drop) it would be easier to compare. Though it's probably worth sticking with the R3 for a bit longer while you adapt your position as the best option may alter if you end up going a lot lower over time.

Attached is an example of how I would set up two different sizes of Evo for myself (given that I've had 8 Cannondales the Evo is a strong contender to be no. 9). Very similar weight dist, same stem length, minor difference in bar reach and a few spacers. If I wanted the option to go higher I'd likely choose the 60 and if I wanted room to go lower - the 58.

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http://www.speedtheory.co.nz

Hm, this is interesting. I was basing off a chart from Tour (page 71 here http://www.tour-qtr.com/epaper_4_2011) which looks quite different from this data, even taking into account I (and probably Tour) used a cycling inseam - in padded bibs. They use a metric of inseam/height, so for me 84/176 = 0.477 and on the chart for my height it's right at the top border for long-legged, short-legged is at 0.43 (76 inseam) and median is around 0.455 (80 inseam).