These little digital scales are available on Ebay. Very accurate and economical and run on AAA batteries. Rated up to 100 lbs so no problem knowing weight on trophy fish.
I have a couple and the one I use has been modified, I cut off the little hook on the bottom and added 2 S hooks to make it stronger and easier to hook weigh sling. At this price ($6.85) there is no reason for people to "guess" the weight of their flathead
I think Kip is right. Flathead are very efficient hunters and completely inhale baits. They are use to baitfish struggling to escape and after swallowing they clamp their mouths tightly to prevent escape.
In lakes our lines are not held tight by current so we rely on flathead moving with baits to tighten lines before hookset. Occasionally we have female flathead that are content to swallow a bait and just lay motionless on bottom. I am not sure if they want to make sure the baitfish suffocates before moving or if they are content with one bait for a meal. It does make for a tense wait until the fish finally decides to move.
Setting a hook on lake fish is also more pro active. We allow the fish to run enough that we know the bait is taken and let them tighten lines before swinging and crossing their eyes. I have never had enough confidence in circle hooks to try them in lakes.