Does it really matter? Barring a chronograph, you'll never really know. I have access to a LabRadar. Hardly, ever use it. I let the target determine the load, based on accuracy.
Happen to own a 2.5 " Bulldog and a 3" S&W. However, never used HP-38, in last 50+ years of reloading. For top velocity, I use either Unique (Skeeter load) or 2400 powder. For plinking, Red Dot or similar faster powers.
I have only chronographed two of my stiff 2400 loads, that I carry for back up bow hunting, in the Bulldog.
265 RNFP (conventionally lubed) with 13 grains of 2400 for 828 fps.
250 RNFP (conventionally lubed) with 13 grains of 2400 for 818 fps.
Based on extrapolation, I would hazard to guess, your load is around 800 fps, plus or minus 25 fps.