Why not trig out the size of the group, and what it'll be on the films you want to use?

That's what I do before shooting a group, and it works well for me. But I use swing-lens cameras to do it, so my calcs aren't your calcs.

But to calc it out, pick a subject height, distance to camera, and the lens focal length. Convert to all inches or all mm, ie, common units. Then figure that if subject height X, at distance Y from the camera, produces a right triangle with angle Z.....that angle Z will continue into the camera, only now distance Y (in the camera) will be focal length, and subject height on film will have the same relationship in the camera (X/Y) that it has outside the camera.

Make sense? No guesswork at all.

And people widths vary, but I figure about 18" across on average.