Your math is kind of right but you are going the wrong way.....as the CA decreases in size the velocity will increase.....ie. too large of a port equals low airspeed...so while the two airspeeds can be expressed as a percentage of one another the calculations need to be reversed to figure CA
V is cfm/A(sq ft)/60

So with a 3.687 CA
V is 450cfm/(3.687/144)/60
V is 450cfm/.026/60
V is 288 fps

With a 2.90 CA
V is 450cfm/(2.9/144)/60
V is 450cfm/.0201/60
V is 373

Given these speeds....2.90 is roughly 78% of 3.687 and 288 is roughly 78% of 373