Warhammer unit sizes?

[Mr Smit]

What is the correct/optimal/asthetic unit proportion to use in Warhammer FB, for archers/thunderers and warriors?

10-18 archers would be 2 ranks? 2x5 up to 2x9

21 or 24 archers would be 3 ranks? so 3x7 or 3x8

4x6 would be wrong surely as they are a distance unit they would want a thinner line for coverage and nor need much depth

Warriors?
18 would be 3x6?
24 would be 4x6?
32 would be 4x8?
40 would be 5x8?
48 would be 6x8?

what about 20/21 20 4x5 looks too square, 21 3x7 is that too long and thin?
I know extra ranks gives steadfast but surely number one is the width, as in 2 more attacks from 3x8 than 4x6