Here is a reliable way to produce STmaps from PFTrack for use in Nuke when undistorting and re-distorting a plate.
This is the method outlined by Dan Newlands on his excellent blog Visual Barn. There you can find many tutorials and methods for professional Matchmoving.
Dan Newlands walks you through the whole process very clearly and I cannot really add anything much to what he shows you. I want to show my setup in PFTrack and Nuke for my own reference.
Here I show the Add Distortion node. Use the 'original clip size' option. The three export nodes are for the undistorted plate (for use in your 3D app), the undistortion STmap and the re-distortion STmap.
The undistorted plate and the undistortion STmap will have a different size to the original plate, but the re-distortion STmap will have the same resolution as the original plate.
Here is the Nuke setup. The first STmap node will undistort the plate. The second STmap node will re-distort the plate to the original state.
Any 3D rendered elements can be introduced between these two nodes and they will be re-distorted to match the original plate. If that is your workflow, then use the undistorted plate exported form PFTrack in your 3D package and match your 3D elements against that.
One thing to note: if you see blocky or tearing artifacts in Nuke it may be that the filtering option in the STmap node is set incorrectly. I have found that 'cubic' filtering seems to work well, although resulting in some softness in the final redistorted image.