I was checking out my torque wrench. I measure it to be
429mm between the pivots, and made out of a 12.4mm steel rod,
that is marked as 150 lb ft when the indicator deflects by 38mm.
The indicator is a little short of the outer pivot---the
deflection at the outer pivot for 150lbf is around 41mm.

I found the formula for deflection: delta = F L^3 / (3 E I)

E is the flexural modulus --- I'll use 180 GPa (Young modulus for steel)
F is force applied,
L the beam length
I is the areal moment of inertia, I = pi/2 r^4

I am going to use the torque instead of force, T=FL, i.e.

delta = T L^2 / (3 E I)
= 150 lbf ft (429mm)^2/(3 180 GPa (pi/2) (12.4mm/2)^4)

'units' calculates that as 30mm, a little too short of 41mm measured.
Am I making a stupid error somewhere?