Using simple trig, assuming there is some temperature gradient accross the beam splitter mount:

Delta Theta/Delta T = alpha*L/h, where alpha is the coefficient of thermal expansion, L is the length of the beam splitter in the beam axis, and h is the distance accross which the temperature gradient exists.

For the aluminum beam splitter, alpha = 2.34*10^-5 at RT and is a bit below that value near 123 K, L = 1.25 in, and h = 4.5 in.

This gives Delta Theta/Delta T = 6.5 urad/K