I bought an air wrench from TSC that takes only 5.5 SCFM air flow that goes up to 550 ft lb; think it's called "Farmhand" or some such. It loosened one of the bolts from my LX4 after a year's use. For the other bolt, I used a 6' pipe put on the end of my 3/4" drive.
Look up on the internet for torque wrenches or torque multipliers. Think you'll find them quite expensive.
I've thought about maybe putting a notch near the end of my piece of pipe and attached one of those old pull scales that go up to at least 100 lb.
When I put my blades back on, JD says to recheck them after 10 hours use. I did, but after another years' use, I took them off again. They weren't that tight. They came off quite easily with my 6' pipe "cheater". Bought new nuts from JD to put the blades back on but haven't done so yet. The type of nuts on it are the ones that go on tight from the beginning. Once removed, they tend to loosen up.
Still haven't solved my torque measurement problem either. Generally, I think you can kinda tell how hard you're pushing. A good push is close to 100#; I'd think. A big guy could bust the bolts by pushing too hard. I'm not that big. I tightened them tight enough such that they didn't seem to want to move any more. That wasn't enough before.
I'll be checking back herein to see if someone else gives you a better method. I'm interested.