The maths checks out. The diagonal of a 3" x 1.8" screen is indeed 3.5", and 3" x 1.8" is in the same ratio as 800x480.

That would make the N900 diagonal 85% of the N810 diagonal, which makes the screen area (in square inches or square mm) around 73% of the N810.

Because of the 800x480 aspect ratio, the screen area of the N900 (5.4 square inches) is 4.5% less than that of the iPhone (5.654 square inches), even though their diagonals are the same.

Regards,
Roger