I have used Trail Boss exclusively for many years now, - in .38 Long Colt, .38spcl, 8mm Lebel pistol, 6.5 Mauser and 10.4 X 38 Swiss Vetterli.
I have never loaded a cartridge up to the base of a bullet, although load data says it would be safe. It is the powder I feel comfortable using in antique firearms if not using black powder for safe pressure.
I load it in pistol using one .7 dipper-full in .38spcl and 8mm Lebel, which seems adequate for the target distances we typically shoot. I believe .3 for .38LC. The Vetterli, without referring to notes, took three .7 dipper-fulls, but I could have gone a tad stouter in hindsight, and I believe about the same in the 6.5 Mauser. Again, seemed to be enough to get bullets out to the targets.
I use a single stage loader and hand fill cases by Lee dipper, so no experience in metering to relate.
I like the simplicity of using that one powder to cover all shooting situations. No need to store a lot of different powders. Granted, I am not the type of shooter who experiments with trying to discover just that right combination of powder, bullet, primer etc. to fine tune performance.
I bought a 5 lb. canister right before the big drought several years ago when Trail Boss was not to be had. It has stored well in the basement and seems to have kept its potency.
RCJ