First off, kbgl suggested using HARDboard, not CARDboard for a screen material. Just thought I should point that out.
I would suggest this material for a screen as well unless there are other reasons you want a fabric screen. Most places that sell the 4x8 foot sheets of tempered hardboard will also cut them to size for you either free or for a buck or two (usually it's free).
I know nothing about the screen material you mention, but I do know it is easier to paint a solid substrate than stretched fabric if you are thinking about rolling the paint on.
That felt tape looks interesting!
The links you gave for the paints are both correct, but be SURE to have the Valspar Ultra Premium Enamel tinted to match PPG Bermuda Beige #427-2
or the mix won't be a neutral gray.
Also, you only need 8 fl.oz. of AAA-F to add to a quart of latex paint to make BW, your link is for a purchase of 16 fl.oz.. If you intend on trying to darken Black Widow to make an even darker screen, don't do so by simply adding more AAA-F; while the mix will get darker it will also move away from being neutral in color.
You didn't say what the ambient light conditions are in your home theater. You certainly have a lot of Lumens to combat quite a bit of ambient light. The PJC Calc is showing 70 fL. with your PJ and screen size. A BW screen performs great with even less than 1/4 of that much light which leaves you a lot of room for lamp-aging, using econo mode etc..
With the brighter PJ's in use these days, it seems it is time to come up with a darker BW!
Kbgl is correct that you might well be served with simply a dark neutral gray paint, and to save some $$$ you might want to go that route. What BW does is produce a physically darker screen (about N7.5) that performs under projection like it was a lighter shade of gray (about N8.2).
In case you haven't read about Munsell N values yet, Pure White is N10 (reflects 100%) and Pure Black is N0 (reflects 0%). N5 only reflects 18% of the light that strikes it (this is the color of the Kodak Gray Card used in photography).