Here's a little article you may like reading.
How NOT to Balance a Blade. A lot of You Tube videos tell you to hang your blade on a nail hammered into a wall. Bad, really bad advice. Think for a moment ... the blade is bolted on to your mower using the center of the bolt hole and then rotates about this center point, doesn't it? So does it make sense to use the edge of the bolt hole rather than the center? Of course not!
To be accurately balanced, the blade MUST (a) use the bolt hole and (b) be perfectly centered in this bolt hole. Now think about just how good a tool is a nail. It has no bearings, so it has no precision nor the smoothness only a precision machined tool with precisely machined bearings can provide. Due to numerous reasons, a nail is a joke. Would you let your tires be balanced mounted off center on the balancing machine? If you are using a nail, don't be offended - just try something better than a nail and you too will laugh at using a nail.
Why not use a "stepped cone" balancer, after all You Tube videos say use these so they must be OK, right? Wrong. A stepped cone has several per-determined step diameters which almost never precisely fit the exact diameter of your blade bolt hole. So like a nail, the blade most likely is off center. And like the nail, this balancer has no bearings, lacks precision machining, and seems to wobble forever. Add to this what you have set the cone atop - is the bench or table perfectly flat so you can get precise readings at the end of the blade? I doubt it, so you probably settle for "good enough". But it isn't - it isn't if you care to keep from ruining crankshafts or deck spindles