If I understand correctly, you are scanning 4x5" negatives, and not the 8x10" prints, right?
However you are doing it you are getting great results. Do you mind sharing a few details about your process? I'd guess Step 1 is something like, "Start with an excellent negative"?
Happy to share: It depends on what and where, meaning, when I am in one of my locations (houses), I don't have my Epson Perfection V850 Pro scanner ($1,299) that I am in the process of scanning dozens and dozens of 4x5 negs of the streets of San Francisco. (I will share some soon). As I go through negatives in said house and find something to share right now, I -- get this -- put the 4x5 neg on a lightbox, take a picture with my iPhone, put it into Photoshop, reverse image, crop edges, size down a bit, and upload. That's what I did with the boy on the scale. I intend to scan it on the Epson 850 later at hi res.
If I have a print but no negative, I will scan the print, to share here, on my Epson Perfection V30, priced very low at Walmart. My next photo, "The Creamery 1948" is that example, scanned at 600 dpi and sized down to share. But the best photos are the 4x5 negatives scanned at hi-res on the V850. And yes, all my dad's photographs are alway excellent!