Well first thing I noticed in these two "images" (the actual chamber) is that they are not the same in design. The Sketchup arch is a standard(?) arch, I don't remember the names of the different arches right now. And the spread sheet is a vertical arch, where the ends of the arch are inline with the lower wall at the start. This is a BIG difference in windows but probably not so much in down baffles.
But the inline arch is a longer arch then the standard arch.
Are you sewing the top to the baffle's? i.e. The top is one sheet and the baffles are all individual? If so I think Sketchup is more accurate. Now if the baffle was made by sewing two baffles together at the top, then I think the spread sheet would be more accurate.
Did that make any sense?
I will see if I can find the difference in eclipses, and the right names.