I think the best size for you in Cannondale is 51.
You will maybe have to change the stems size or the seatpost to be perfectly positionned on the bike.
You always have to take the littlest size between two different on.
And in regard of the Cannondale geometry, it ll be most aesthetic .
Enjoy your ride
Caad 8 is a very frame in aluminium, there is few years professionnal Team Liquigas used this frame for the race !!
For the size it s always the littlest one who must be choosen.
Because you will minimize the weight
Because you will have a Bike more nervous, reactive
Because it s easyer to change stem & seatpost if the Bike is too big, than cut the frame !!!
Any case the size 51 is a Max to my point of view ( i change my Bike so often, you cen trust me)
Try the web site it will May be helpfull