Hi Mike,
With the majority of my projects I use targets with Ground Control Points so that the model is in world dimensions. Since I know the actual dimensions of the object, I know about what dimension I want a pixel to be. Lets say my units are meters and I want a pixel dimension of about 1 millimeter. I will either use the Unwrap style Fixed texel size with the Texel size set to 0.001 and Unwrap, or I will use Unwrap style Maximal texture count and change the Maximal textures count and unwrap until the Texel size displayed in the Selected model(s) dialog is about 0.001. Often times the Optimal texel size will be a lot smaller than 1 millimeter and would be overkill.
The Unrwrap Style Maximal texture count is the one I use most often when exporting simplified meshes. Here’s an image showing the dialogs for Maximal texture count and the Selected model(s) for one of my projects. Hopefully it will make the previous discussion clearer.
When you export your model what Tile type are you using? I believe modo supports UDIM, you might want to try that if you haven’t already.
Another thing I do in my workflow is build the initial model in Normal Detail without a Reconstruction Region. I then use Set Reconstruction Region and use the Filtering Tool to remove the unwanted outlying area. This removes large polygons (and the closed box). I then use the Simplify and Unwrap tools on the resulting model.