The minimum pad width on the J4/F35 combo is 140mm more than the extension width. If you've jammed the bracket against the stem you might be at 70mm for extensions - meaning 210mm is the narrowest the pads can go.
If you fit an Ergo armrest that increment becomes 62mm as the narrowest.
So you could now widen the extensions to a more reasonable 85mm and have a min pad width of 147mm.
Further, the shape of the Ergo is such that the centre to centre measurement is actually where you contact the pad (unless you really push against the outside of the cup). Whereas the flat nature of the F35 means that the centre measurement is somewhat inboard of where your arms will lie. I can't find a drawing of the F35 to demonstrate, I'd estimate the actual contact to be 10-20mm outside the pad centre.
Thus, in effect, the Ergo armrest will get you 100-120mm narrower than the F35 and provide finer adjustment for refining your position.