LSM9DS1 A 9 degree of freedom Inertial Navigation Chip

That's a cool hack. What we call Q&D (quick and dirty).
Aran2 gives -180 to 180.
theta=atan2(z,x)
if(theta<0) theta+=360. Then theta ranges from 0 to 360
For the left handed axis on this chip theta= atan(-z/x)

Using the accelerometer readings gx, gy, gz
theta = atan2(gz,gx), phi=atan2(gz,gy)
rotate around yaxis
x1=gx, y1=cos(theta)*gy-sin(theta)*gz, z1=sin(theta)*gy+cos(theta)*gz
the rotate around the xaxis
x2=cos(phi)*x1+sin(phi)*z1, y2=y1, z2=-sin(phi)*x1+cos(phi)*z1
At which point z2 should be -1. X2 and Y2 should be zero
If the rotations are them applied to the magnetometer readings the compass can be leveled.
Some gotchas:
May need a minus sign in the atan2(-a/b)
May need to rotate the opposite direction by changing the sign on the sin terms( +sin becomes -sin, and -sin becomes +sin)
I captured some data earlier this morning and did some of this in a spreadsheet. It looks like it works but I want to finish the mounting fixture and change the code to load the zero calibrations rather than recalibration on each run.

                     X  Y   Z   atan2(x/z)  atan2(y/z)

Acceleration -0.000249863 0.003465652 1.000553131 -0.014308162 0.198456693