It can be simplified this way:
The OpenXR render scale controls the display size exposed to software using this stack. In this case MSFS2020.
The MSFS render scale is a percentage of the device display size to use for rendering the game graphics.
Thus, when both are at 100 (%), OpenXR exposes your full headset resolution to the game that, in turn, renders the graphics at full resolution.
If OpenXR is set at 50% and MSFS2020 at 200%, OpenXR would expose half your heaset resolution to the game that, in turn, would render its graphics using a doubled-size resolution and then scaled them down to fit.
You get the idea for the almost infinite possible settings.
Now, is there a better setting? Yes and no. It’s extremely dependent on your hardware performance. If you don’t have a latest generation monster card, you have to lower down the resolution to gain frames per second.
I am waiting for a newer graphics card (like many!) and until then, my Radeon 5500XT is pumping graphics with OpenXR at 60% and MSFS2020 at 80%. It give me sufficiently smooth VR performance with almost all settings at High.