I think there's a whole range of height/width ratios. Depends whether your forest is twisted alpine or coastal trees of represents a tall, straight sheltered forest. Also whether your setting includes some open space on one or more sides or the trees go right out to the edges of the tray so it is not possible to define height/width ratios for all group plantings.
Ratios will also change as the tree trunks develop. Bonsai rarely stay looking the same forever.
If in doubt, start a little shorter because trees tend to grow up and out so nearly always end up taller than you first planned.