The chart I had from my GP years ago (2002) is this:
ImmunoCAP RAST (units are kU/L)
Class 0 = <0.35 (absent/undetectable)
Class 1 = 0.35 - <0.7
Class 2 = 0.7 - <3.5
Class 3 = 3.5 - <17.5
Class 4 = 17.5 - <50
Class 5 = 50 - <100
Class 6 = 100+
(Multiple health sites have the same numbers.)
I remember reading and just looked up the values for 95% negative (unlikely allergic) and 95% positive (likely allergic) for different allergens. 95% negative chance of allergy level for all listed allergens on the list was 0.35 (pending reaction history). 95% positive chance of allergy levels were: Egg = 7, Peanut = 14, Milk = 15, Fish = 20, Soy = 65, Wheat = 80. So maybe the number for 95% positive for almonds is higher than the number you received on your test, which could be the reason they want to do the challenge?
Best of luck.