Let it stick out. Honestly, my anal self noticed it at first and wanted to fix it. Now I don't notice or feel compelled to resolve the issue. Per
@mrblaine:
"There is almost nothing you will do that will look better than the gap. I trim on the inner lip that faces inward to the top of the backer and call it good. If you notch the flare, that lower end will never be tight to the backer or hasn't for all the ones I've seen. The obvious answer is a piece of ABS that is 3/16" thick that goes behind the flare everywhere it is on the body. Getting the edge of that and the flare to line up nicely is a nightmare."
Now having spend 4x as long screwing with getting them to look right as it took to actually R&R them, I think I agree.
With the RC's I ended up losing most of the connection with the inner surface, so it's pretty much just covering the end of the slider at this point.
I thought about finding a piece of plastic the shape of tiny angle iron and epoxy it in to restore that structure but haven't had time to look for anything yet. It would also help fill the gap I ended up with, which is where I took too much and I'm flush with the tub but not the slider.
This is the best one, which is the last one I did and I accomplished by basically taking shaves at a time and test fitting until I was happy with the fit.
This is the first one I did, which is the worst because I took the most accurate measurement I could manage and tried to take off the exact thickness of the slider, which was too much. Pay no mind to where I scratched the paint by not noticing where my masking tape had moved during initial slider fitup.
I don't like the way the rounded over edge where the flare meets the tub reveals the old chipped and rubbed paint that was hidden behind the OEM flares, but it probably made the results of this trimming job turn out somewhat tolerable vs what I would have had with the sharp edge of OEM flares.