I removed the cosmetic pieces on each side of the upper firewall as shown in M-7. Then I leveled the fuselage fore and aft, and side to side.
When I was leveling the center section spar to the wings and subsequently to the fuselage, I had bondoed two small leveling boards on top of it (Chapter 19 - Section 12). I decided to use these leveling boards again for positioning the upper firewall. I hot-glued two squared boards on top of the leveling boards with its aft edges at the same plane as forward surface of the bottom firewall. Then I hot-glued the upper firewall in place. Actually I did this before installing the TB in the previous section - to ensure the TB fits well with the upper firewall.
I measured the back side of the upper firewall with my digital level to make sure it is perfectly vertical. In addition, I hot-glued 2 small bubble levels so that I can verify its angle position throughout this Chapter. Note the miniature yellow bubble at the bottom edge...
Once I am certain of its position, I removed the firewall and added wet flox at the contact points between the firewall and the longerons.
After the flox cured, I double checked the vertical position of the firewall with my digital level. It was 90o vertically from left to right until I got 3/4 of the way to the right, then I got 89.9o. Pushing the top aft just a little bit got me the 90o - good enough, I think.