From MC true momentum values and a magnetic field value, one should be able to calculate the radius of a helix as
pt = R * B * 0.3

When I look at MC true momentum values, and the magnetic field value of 4T, as it should be in the simulation, I get radii that are approximately 10 mm larger than the radii I get from fitting a circle to 3 spacepoints in the x-y plane. I am using the same method as was in G4MICE (matrix determinants), double checked against book sources. I've attached a histogram for you to see the residual for R_MC - r_recon

I'm using MC px and py at doublet layer 0, from station 5, of tracker 0 and B = 4T to calculate R_MC. I reconstruct a circle in the x-y plane using spacespoints from stations 1, 3, and 5 in tracker 0, and find the radius of this circle. And then I do MC R - r_recon. There are a few outliers in the plot. I will be thoroughly checking this out, but at a glance they seem to be from cases where there is either more than one spacepoint to choose from per station, or where the MC true momenta don't look appropriate (i.e. large increase in MC pt from tracker 0 > tracker 1, or large decrease in MC pz from tracker 0> tracker 1.) I've included a log-ish type of file that contains all of the residuals where MC- Recon is < 0 . Can't include the entire input file because it is too large.

I don't know whether its the matrix calculations, bug in my code ( also unlikely since its basically the first thing I do) OR if there is something awry in the simulation.

It should be noted that all other track parameters will depend on the radius, so if this is calculated wrong, there is no hope for the rest.