Figuring 45 ppg each from the DME and the sugar (rough approximation), one pound of sugar or DME per gallon yields a gravity of 1.045. My fast dime store math, 45 ppg by 1.375 total pounds for 61.9 gravity points per gallon, or a gravity of 1.062 in one gallon. Assuming that the sugar attenuates fully, and the DME attenuates 75% (call that average attenuation), that's an FG of +/- 1.011 (zero gravity left from sugar, and 25% of the points from the DME), and then (OG-FG)*131.25 yields approximately 6.5% ABV. If the DME stays the same and attenuates the same, and the sugar still attenuates fully, FG might stay fairly similar, at approx 1.011. For 8%, work it backwards. 8 = [((45+45x/16)/1000)+1)-1.011]*131.25, where x is your oz of sugar. I get about 9.5 oz sugar on top of your 1 lb DME for 8% ABV.
Of course, this is basic dime store math, with a lot of assumptions made, and if these other folks are using software that more accurately quantifies the things I'm simplifying, then I would go with their figures.