If 704 is related to 640, then 640/2.35=272... if you have a picture 274 pixel height, then 274x2.35=643.9

So 704/640=1.1 then 643.9x1.1=708.29 - that is (a bit) nearer to 712 than 704... (^^,) - so, at least you should crop at 708 than 704.

Also, dicrepancy is tiny, so I'd prefer to have a really small error in aspect ratio, retaining more picture, than having a "right" aspect ration, losing some details that could be precious - as much as a little bit than 1% could be considered precious, of course!