Not an expert acoustician, but an engineer and I love math!
If you like what you have, blueprint what you have: seat location in the room, speaker locations, speaker to seat measurement, tweeter to tweeter measurement, L<W< H of room, etc.
There are sometimes conflicting items to juggle but I suggest getting the speakers placed in the room per one of many techniques available. I used 3 of them and they all gave me the same locations within inches. Vandersteen has a well written guide that mimics the videos above. It will give you front wall and side wall locations.
Then, your seat relative to the speakers is important and it can cause some confusion, but the 83% value is a ROT, not a hard law. Tweeter to tweeter distance divided by the tweeter to ear distance at 83% is a good indicator you are headed in the right direction.
Then measure to see where your seat is and another ROT is your seat should be 62% back from the front wall. This helps to avoid peaks or nulls which based on the room length is always going to be mathematically at halfway, quarter of the distance, etc. This 63% does not guarantee magic. At that point if your speakers are pretty close to both approaches, you are looking good.
As mentioned above, do you have any smearing in the treble or echoes going on? Are you using any first reflections absorbers or diffusers? Smearing is often treble gone wild and bouncing all around and can be distracting. You like your sound so perhaps it is under control.
You are curious about your room response but it is a Pandora's box once you start on that path. We typically cannot make the room response flat, but can make it flatter. Not sure where I heard it, maybe from a speaker designer, but a + - 5 db room response is deemed to be pretty good from 20 to 20khz. Most of the problems will be bass related and bass trapping can have a very nice impact on that end as well as draw the midrange into compliance as well.
It only takes 20 seconds to run REW if you have the key components like the mic and a preamp and soundcard. The setup is critical in the REW software and once you get it, then you can always reuse those key controls. The REW start up guide will list some gear like the mic and preamp and cabling to hook up to your stereo preamp to send the signal out for the room mic to pick up. A cheap tripod is handy, and put the mic almost vertical in your chair where your ears are normally and go for it.
Also, try AMROC's site and punch in your room dimensions and you will see all your room modes as you slide your cursor up the piano keys. Remember, the room is like Mother Nature, it always wins and we have to do the best we can to deal with it. Compare your seat locations with some of the room modes driven by the length, width, and height and you can quickly see where you may be dealing with some monster problems and have unknowingly accustomed yourself to those.
I hope you get REW running someday but remember it will look very ugly and can be like staring at a mountain to climb but knowing what is going on and how to treat your room will give huge benefits. Check out my TN 2023 system. I just reworked the bass trapping again and added ceiling treatments and while the response did not show major changes, it sounds even better and makes me weep.
Then the cycle starts over and you crave better gear again! ha