Can someone correct me if I'm wrong with my calculations?

The calculation I'm using to determine necessary lens focal length to achieve a minimum of 112 inches print length is Lp=Dp/Fl * Lf, where Lp is "Length of print", Dp is "Distance from film to print", Fl is lens Focal length and Lf is Length of the film. I have at least 4270mm distance from film plane to easel... a tiny bit more really.

Here's how I calculated for 56x112mm film...

Lp = 4270mm/135mm * 112mm
Lp = 3543mm or 139 inches

And for 8x10" film...

Lp = 4270mm/300mm * 244mm
Lp = 3473mm or 137 inches

I realize this formula isn't absolutely precise but it's close enough, right? That extra 20 inches print length is a bit of built-in fudge-factor or, better yet, ability to crop a bit if necessary.