Altitude was 9,500, speed at 150 true, 9.9 GPH, 58 percent, manafold at 19.8 percent, prop at 2390, CHTs between 331 and 364, EGTs between 1325 to 1360, LOP was -43 to -64 degrees. Engine felt good.
I have an IO 540 with 9:1 jugs. The plane is unpainted so that may account for the loss of a few knots. I have a Garmin G3X system and used the lean assist. My cylinders run 360 to 390 ROP. I showed -43 LOP on the last cylinder to peak, as my LOP setting. OAT was 10c at 9,500.
Question, how LOP do you go, -20, -30, -40 or ?. What power setting do you see. Are you looking for any particular numbers when setting to LOP. I was at 58 percent, is that about right or should it be set higher.
Let's tidy up a few fundamentals before moving forward.
Manifold pressure units are not a percentage. It is expressed in inches of mercury (abbreviated ""Hg"), typically measured at the intake port of the right rear cylinder.
You did not mention throttle position. We would generally assume you were at wide open throttle at this altitude. However, 19.8" Hg is a little low as compared to some other Vans examples. Could you confirm throttle setting?
Your exhaust gas temperatures do not matter in absolute terms, so knowing they were between 1325 and 1360 is useless information. Here the only EGT information of note is where you are relative to peak, and you've noted -43 to -64 degrees. As others have commented, there is nothing wrong with being that lean, but it puts you pretty far down on power, so airspeed will be reduced as compared to what most RV-10 owners post as normal LOP cruise.
Which brings us to your "How far to lean?" question. The answer is "As lean as you wish", the limits being (a) unacceptable loss of airspeed, or (b) engine roughness. What is practical? Lycoming has always suggested peak EGT as the economy cruise setting. Despite much rock throwing (at Lycoming), you'll note that LOP devotees very often suggest something like a whopping 10 degrees LOP at altitude.
Neither is wrong.
At 9500 feet you should be at wide open throttle, and if so, the only way to increase indicated percent power (you reported 58%) would be to increase RPM. Again, there is no "wrong" RPM within the practical range one might select for cruise. More will require higher fuel flow, less will require less. You'll go faster or you'll go slower. Almost everyone has a favorite setting, usually based on comfort (like perceived vibration) or old belief (the way they always did it). Others enjoy mapping every possible combination, then developing an operating guide based on reams of data. That's cool too.