Guessing is unfortunately the best practical recommendation I can give for operating the Lens module.Lens distortion
can be an incredibly hard thing to model and reverse, especially when dealing with different orientations, circumstances, flexure and configurations across different nights.
When dealing with just a single, well built lens that always sits flush with the imaging plane and dealing with a CCD that doesn't change orientation, it is possible to model the (typically modest) distortion once and re-use it. This is what applications such as LensFun and Adobe Camera RAW do. Note that it is then necessary to apply the correction to very sub frame before stacking (given that the field will likely rotate between frames).
However for prime-focus astrophotography (as most of us practice it) it is very hard to model a distortion that is always true/valid given the many factors that may change the distortion from night-to-night in modest equipment.
The Lens module in StarTools employs Brown's distortion model to correct for "simple" radial distortion, which can alleviate some of the worst effects of coma. It is however a sub-optimal solution - the best solution, as with all things, is to address the issue during acquisition by using a coma corrector.
The one tip I can give however, is to apply the Lens module early on, before cropping any significant portions of the image (except stacking artefacts).
Hope this helps!