Given that stubby little LCR barrel, 611 fps may not be too far off the mark. As I said, 3.4 gr Trail Boss behind a properly sized 125 gr cast lead bullet (in .357 cases) averages 755 fps from my pair of 6.5" Blackhawks. Losing 25 fps per inch of barrel length is not unreasonable.
With this much or more Trail Boss in .38 cases, you need not worry about position sensitivity. Those cases are plenty full. That's what Trail Boss is all about.
I have a hunch where some of your power loss is going, though. You are using Rainier "ccrn". Pardon my referring to them earlier as "copper clad". It's probably not proper to refer to thin electroplating as "cladding". Ranier's standard sizing for that bullet is 0.357". Now here's the thing, I own 4 Ruger revolvers, and all them them have very large cylinder throats. My two Blackhawks in .357 mag, and my SP101 in that caliber, all have cylinder throats of about 0.359"! Yes, they're that big! They're very consistently sized, but they are large. My .44 Mag Bisley is similarly oversized.
Now I shoot cast bullets, so I am able to order custom sizing. I get them sized to 0.360" when I can, but accept nothing less than 0.359". I do this to optimize for accuracy. If I'm wrong, I hope somebody more knowledgeable will step in and straighten me out, but having a bullet sized smaller than the cylinder throats will reduce pressure. Lower pressure results in lower velocity.
Any revolvers with cylinder throats as big as mine will shoot STANDARD SIZED BULLETS slower than most published load data, when corrected for all variables such as barrel length, primer, COAL, etc. etc.
To better utilize the load potential, you will need bullets sized to your cylinder throats (or a thou' or so larger). That's hard to do unless you're shooting cast lead bullets. But there's much to consider when moving to cast lead, not the least of which is you first want to remove all residual copper from the bore.
And your LCR is a defense gun. Maybe not the best gun to be experimenting with for cowboy action type loads, which is what Trail Boss is for. But if your objective is to produce powder puff loads for your LCR, there's probably no better choice than Trail Boss. Oh, and when you think about "minimum speed" to ensure you don't stick a slug in the barrel, remember that you can reduce that by about 25 fps or so per inch of barrel length, when considering this class of loads in revolvers.
I have done this before and it is one of those unnecessary things in life that's for sure. Then if you miss it for whatever reason the next bullet will be really dangerous.
