Flaggers and Control are apparently having some problems making out car #'s. They did not mention SpecE30 specifically. The NASA CCR says the numbers on the side should be at least 10" tall. Our CCR says 8" and class CCR supersedes.
Off the top of my head I can’t think of anyone that has only 8" #'s on the side. But if you do, I’d supersize that # a bit even tho you’re within the rules. Also, if there isn’t A LOT of contrast between all 4 of your # decals and everything else, don’t be shocked if you get told in Impound that you need to find some more contrast before the next event.