I am looking for someone who can go beyond what the NWFA is willing to advise on for the installation that I'm hoping to finished this month (January) in Southeastern PA.

The NWFA suggests that "To minimize expansion on floor spans
wider than 20 feet, spacing between rows may be needed"

They go on to explain the expansion calculations. In my case, 6" flat sawn red oak that will be laid across 20'. The width of the board will span the 20' length of the room due to subfloor orientation.

Current RH/temp is 35 and 60deg. Summer worst case RH and temp estimated to be 50 and 80deg. From the EMC table that gives me 2.1% MC change.

So red oak DCC 0.00369x2.1x20x12=1.86" of expansion across my entire 20' floor span (is that right?), considering I'm on the one temp/RH extreme right now.

But here's where I feel the NWFA falls short. They explain the calculations, then state "Where additional internal spacing is required
with some solid wood floors, use washers". Nowhere does it quantify what amount from the result of the calculations require internal spacing. Does 1.85" fall into that category? How much of that expansion will be stunted by the staples. And how much is covered by the 3/4" perimeter expansion gap that will be installed?

Ultimately I'm trying to figure out if/and how many inches of washer rows I would need to avoid any cupping issues come summer humid season.

Thanks!

Forgot to mention these boards have acclimated for 10 months now, ha! So you can see why I don't want to wait til spring or fall to install.

NWFA members have requirements on the correct width when the flooring is milled when it is kiln dried. So, starting out at exactly 6 inches as it starts out. After 6 months of acclimating you may notice a difference in width that will help account for some of the presumed expansion at your equilibrium width for the middle of the Summer/Winter value for your area.
Start by laying 10 boards together on a flat surface to see how much the width is now. You can predict how many washers you will need to make the total projected expansion over the total 20 foot width by dividing your ten board width by 10 , then divide that into the 20 foot width to see how many rows and how much you need to add over the width. Washers are usually an eighth inch so you can find how many washers you will need to get to the full 1.85 inches at the wall. Space the washers in rows across the floor leaving out the last couple rows of washers so you take the expansion gap at the edge into consideration.
The washer spacing may look better if you consider where the furniture may be placed as there will be some time for the washer space to be filled in by the time the floor is acclimated. The final width of each board can be averaged out according to the specification from an engineering handbook. I find that if you use a heavy rubber mallet to tap the floor near the expansion gap rows after the washers are taken out it will help the flooring to move to where it will eventually rest after acclimation, which may take a little or a lot of time depending on the width of your flooring as it is being laid. Since it has been acclimating for six months, already. If your flooring has already increased in width enough so you don't need washers you will be in luck. I've seen washers sell for a dollar each at some stores. You just need enough for one row at a time, then pull them out to re-use on the next row according to where they need to be spaced.