Using a CRP-5, set 8000ft against +15C. Then look at the inside scale for 85kt IAS. This gives you approximately 99kts TAS.
Then, using the 1 in 60 rule:
(Height/Distance travelled)x100=Gradient in %
so: (Height/10032ft)x100=3.3% (you get 10032ft from (99/60)x6080)
Height = (3.3/100)x10032
Height = 331ft - near enough to your given answer.