SAM Incidence Angle Calculations

Good Afternoon,

I'm having trouble deciphering the single axis tracking code in SAM. Specifically what I'm trying to do is model the Utility PV plants in the Solar Integration Dataset. Here's the basic equation I'm using:

ARG = sin(zen) * cos(azm - sazm) * sin(tilt) + cos(zen) * cos(tilt)

if ARG = XYZ then

blah blah

end if

inc = acos(ARG)

This is a horizontal mount system with a north to south orientation. My assumption are:

sazm = azm
tilt = 0

I've tried moving these assumptions this way and that but nothing is coming out correctly. I've read the Technical Reference documents and I've experimented with alternate INC calculations. I've double checked by zen and azm calcs and they come out fine.

Note also that my calculations for Fixed systems are coming out fine as well.

Can someone point me in the right direction? Is there a forum related to the Open Source release of SAM on GitHub?