Yet, this is a good example of where the "rule of thumb" is not applicable.
My coop is another example.
If the birds can come off the roost and hop down into the larger area, that area is what is applicable for coop space (until released) provided the 1 linear foot of roost space can be met, which an 8' long tractor meets.
I'm thinking of the low-headroom edges of the footprint -- where a hen can't stand because the roof is lower than she is tall.
How wide that lost space is depends on how steep the A is.