Well, the smart butt in me says to tell you to "Ask Al," (it's an inside joke) but in reality, I don't have an answer for you on what "good" compression would be on a flathead. Considering that most engine builders use low tension rings on the flathead, a compression test will tell you very little (in my opinion.) Most 4 cycle small engine builders will be more comfortable using a leakdown tester and testing for a percentage of leakdown in the cylinder. When leakdown increases by 2%, or you hear a definite leakage past the rings or one valve or the other, it needs addressed.
Again, just my opinion, but I think that it will be shared by most flathead engine builders.
Don't really know the answer to your question, sorry. Another thing, I've never tested an engine that hot. Warm maybe, but never hot.
We strived to test under identical conditions, whatever they were.
We also used an electric starter.
If you use the pull start, pull several times, until the gauge reaches a peak.
I don't want to discount the importance of a leak down test, but do you have time at the track to do it? Or is it quiet enough at the track?
And I wonder, are low tension rings better than high compression? And how much compression do you lose with low tension rings? I have always gone with this; Compression Is the Holy Grail.
I certainly don't know it for a fact, but low tension rings don't sound right to me, not if they lose compression for you.
Rings wear, and as they do, I would think, they would lose tension. Would that be about the same as starting with low tension rings?
Bottom line; check the compression after the heats. Keep a record, and if it starts going down, start thinking about a rebuild.
Flatheads use 3 rings (per the rules.) Reduced friction and drag are king (on the bottom two rings.) We often use undersized, machined, detensioned, etc rings on the bottom two. Even on the compression ring, we use a lower tension ring with more material added to ends of the rings to close the endgap down as close as we dare get to zero. Along with that, rings like the Burris compression ring (legal) have a compression chamfer built into the top inside radius of the ring so that under compression it will expand outward against the cylinder to seal tighter. This also helps control ring flutter that is often associated with low tension rings. While Total Seal gapless rings are not legal in the stock classes, even when we've tested them, they hurt the power every time. While it did make less leakdown than conventional / legal rings, (and assumingly more compression,) the power on the dyno was just not there. Low tension rings won out every time. Low tension rings have always sounded right to me, even though they lose compression for you.
This is the popular set-up for flathead rings and has been for several years now. Could there be something else out there that is better (and legal)? Doubtful, but certainly possible. With all the experience and years of R&D that hundreds of professional engine builders have had with the flathead, there are less and less stones to turn over and discover newfound power.
I'm always willing to hear new ideas though. Especially from experienced 4 cycle engine builders.