The toilet fills and the float works but after the water level reaches 1” below overflow tube the float is all the way up but the water still drips very slowly into the tank.. In about 30 minutes water covers covers the top of the overflow tube and drips down the tube continually. This toilet has a donut shaped float around the fill valve that rises and stops incoming water except for the drip. Flapper works well with no leaking around it. I’ve adjusted the adjustment screw so the main flow of water is stopped about 1 inch below top of overflow tube. No mater how much I turn the screw water still drips until it is dripping over the tube. How do I stop the water from dripping and then eventually going over the top of the overflow tube?
The toilet fill valve inside the unit is failing. Now, what to do?
If by chance I had the packaging or documentation from the last time I replaced it, or there's a make/model visible on the valve assembly, I'd see if I could find a replacement washer set. Some fill valves are easily repairable (if you can find the correct replacements). Many aren't, so I wouldn't spend time worrying if this isn't true.
If I didn't have that and it seemed to be a "standard" toilet, I'd check at the home improvement store for a replacement fill valve assembly. In most toilets, replacement isn't very difficult. Drain the tank, unscrew the line from the tank, unscrew the nut holding the valve in place, and replace. The only tricky part is that there's no completely standard assembly. While most standard toilets will be easy to replace, there are some odd bits out there. But this tends to be a pretty inexpensive part for a DIY repair (probably around $20). Just keep all the old parts around in case you get to the end of the assembly and find the new part doesn't fit for some reason.
If you have a custom toilet (very low profile, odd tank shape, etc.), I'd take several good photos of the assembly and go to a plumbing supply house. Ask them if they can identify the unit and suggest a replacement.
Many times I have found a simple flush of the valve itself is alk that is needed. Most valves can be flushed by turning the supply off. Flush the toilet to relieve the pressure in the line and drain the tank. The top of the fill valve on many models can be removed place a cup over the valve top , turn the water on for a few seconds then shut off. Reassemble the valve and turn the supply back on. Most of the time there is a buildup of scale, rust or small gravel that causes this problem and a quick flush (pun intended) cleans out the valve.