I have an older toilet from 1971. The tank holds a lot of water. I took an empty two liter Powerade bottle and put an inch or two of sand in the bottom. I filled it with water and screwed the cap on. I set it in my tank, away from the flush mechanism. My tank is so large that I was able to put in two bottles and it still flushes efficiently.
My father chose another route. He turned the float until the water line was an inch lower. He said at first he lowered the water line too much and the toilet plugged. You may have to experiment for either method.
This leads to my first admission of a “Frugal Folly”. A “Frugal Folly” is when a tip is not followed correctly and it ends up costing money instead of saving money. My mistake was filling the bottle with just water. Without the sand to stabilize the bottle, it fell over, blocking the opening and allowing the toilet to run for a long time! Learn from my mistake – don’t forget the sand!