The Shimano 8 speed hubs are not a math exercise. They are real mechanical devices, with real gears that have to have an integer number of teeth that have to mesh with other gears that have to fit inside the hub's form factor.

First gear inside the hub ratios the chainwheel/cog by 0.527. Second gear increases this by 1.223. Third gear increases that by 1.16, and fourth gear increases third by 1.139. Fifth gear is 1.175 of fourth gear, and one-to-one inside the hub. Sixth gear engages the same planetary as second gear, so it is 1.223 times fifth gear. You can guess then that seventh gear will be 1.16 of sixth, and eighth gear will be 1.139 of seventh, 1.615 of one-to-one and 3.066 times first gear.

tcs

PS - If you're really not using the bottom two gears, put on a 16T cog (the smallest that will fit on the Shimano 8 IGHs) and shorten the chain. If that still doesn't get the gear range high enough (although I think it will), you'll have to spring for a larger chain wheel.