Facts about primary collimator:

Some information about the acceptance of the current primary collimator
(shown below):

Here are the polar angle acceptances along the target:

upstream end: theta = 6.0 to 9.0 degrees accepted,  delta theta = 3.0 deg.
middle      : theta = 6.8 to 10.2 degrees accepted, delta theta = 3.4 deg.
downstream  : theta = 7.8 to 11.6 degrees accepted, delta theta = 3.8 deg.

A plot of the polar angle acceptance is available here
primary collimator extended target theta
acceptance
and below:

Comparison of Monte Carlo to a simpler calculation 
using the above polar angular distribution combined
with cross-sections and Q2 from Greg Smith's spreadsheet:

Spreadsheet calculation:
rate per detector = 724 MHz, average Q2 = .0264 GeV2

Monte Carlo result:
rate per detector = 674 MHz, average Q2 = .0268 GeV2

I suspect the remaining difference is due to the fact 
that in the Monte Carlo calculation, the outgoing
scattered electrons can bremsstrahlung in the target.
So they can scatter out of the acceptance and lower
the rate.  We are re-running the simulation with the
hydrogen set to vacuum.