A rectangle measuring 2.5 x 4.5 won't give you a 60º diagonal...ever. The side measurements of a 30-60-90 triangle are always in the ratio of 1:√3:2 (iow, side opposite 30º is 1, 60º is √3, 90º is 2). Your rectangle would have to be either 2.5 x 4.3 or 2.6 x 4.5.