Inches could be inches of mercury. 1 atmosphere is about 30 inches of mercury. But it would go backwards for vacuum so atmospheric pressure is 0in. and complete vacuum is 30in. What I worked out was for a complete vacuum, even tho we can't quite get there, and the pumps are probably horribly inefficient, but that's just a technological problem.

That would mean 5" takes 5/30 = 1/6 of the energy needed to get complete vacuum on the same volume chamber.