There's a huge difference in visual clarity between 1280x800 and 1920x1080. At 100 inch diagonal, 8 feet should be alright although I would recommend moving back just a tad bit more if possible. It really depends on the person though. I use a BenQ W1070 @ around 160 (I have moved it multiple times between 150 and 170 and it looks to be around 160) and sat on a couch around 17 feet away and I feel as though I could have moved closer for an even better experience but then I wouldn't have been able to fit as many guests into the viewing area.
if you need to adjust the projection down to 90 or 80 inches, is it going to make or break your life? If not, go for it. If you need to enlarge it, go for it. Adjust it to your own needs, eyeballs, and neck.
Just bought and put up an ES Aeon Borderless 100 inch screen with an Benq HT3050. 8 foot is too close. I am at the 11-12 foot mark and it's perfect for 100 inch. You might want to go 92" or 84". And yes the HT3050 was worth the extra $200 for additional keystone alignment and especially for the already calibrated to Rec.709 setting. I hate calibrating any more so this projector foots the bill and is great out of box. This pic is before the screen was up on a dark blue textured wall. Haven't had time for new ones yet, but you get the idea. (blurry on Rick cause of motion capture)
There is no SDE with my setup. The screen is Cinegrey to enhance color and blacks. Just used the google search page, white, white everywhere! No SDE. That's the thing with projectors, screen is just as important. I have always seen the best imagery with entry level projectors combined with a Cinegrey or grey tinted screen. I have been around Benq projectors for a long time helping family and friends with theaters, and they have the better warranty and better quality in my eyes with more options at different prices levels for everyone. My brothers Benq 1080ST has been running strong for 4 years now, no problems just bulb replacement. Setup a W8000 for a friend last year and that projector is truly on of the best 1080p's on the market with lens that can be changed.
Anywho, if you can afford it the HT3050 really nice for a no hassle setup. If you want to shave off the preset calibration, the HT2050 can be had for roughly $700.
Sorry couldn't get back to you earlier, working nights.