Doug.
do this. Lay your tapers on top of each other (on paper, drawing) As you layed them into the bow.
One has a butt of .100 the other .120. The tapers are 36" long (it actually doesn't matter how long they are). At .002 taper per inch, each one loses .072 in the run. Subtract .072 from the .100 and the calculated thickness at the end is .028. On the other one .120 minus .072 is .048. lets add up the thickness of each end when layed up together...
.100 + .048 = .148 .120 + .028 = .148 Unless I figured wrong, that makes it a parallel piece.
When calculating the weight pull in terms of thousanths of an inch thickness, inverting that taper as you did made the difference between .278 thickness at the base and .350 thickness. That will certainly make a big difference. Taking a 1/4 " off the sides of that bow, especially at the riser may affect it 5# but I really doubt even that.
Calculate as I did and see if that makes any sense (and if I added correctly). I know Bingham's talks about reverse tapers, but it makes no sense in my mind (for a longbow). You want the tip to be lighter in weight so it responds quicker, not heavier.
Guys...if I did this wrong, please jump in cause I can always learn, but I think this is right on.
ChuckC