Actually, Dave you had a higher pressure ratio (manifold to ambient) at 10,000 feet with 26" boost than at sea level with 36".  At sea level the pressure ratio would be 36/29.92 = 1.20.  At 10,000 ft ambient pressure = 20.57 " hg , so the pressure ratio would  be 26/20.57 = 1.26.  Not much greater, but it was higher by approx.1.23" Hg. (1.20 -1.26 = .06 *20.57 = 1.234" Hg).

Ed A