There is an empirical relationship for thrust versus altitude. It is based on the fact that decreasing air density causes a decrease in thrust i.e. there is physically less air to compress and expand hence the thrust drops.
The simple equation for this is taken from:
Martin E Eshelby, "Aircraft Performance - Theory & Practice", Arnold, 2000
F/Fsea level=a^x
F= Thrust at chosen altitude
Fseal level = Thrust value at sea level
a = air density ratio (known as sigma)
x =0.7 in troposphere
x=1.0 in stratosphere (~36000ft)
Page 6 in this link will give the the density ratio for different altitudes:
http://home.anadolu.edu.tr/~mcavcar/common/ISAweb.pdf
So for 3500lb of thrust at 40,000 ft we have:
F = (3500)(0.2462)^1 = 861.7lb
Of course this is just an empirical relationship but can give you a rough idea.
Hope it helps!