Ah the toilet bowl. How much time spent there doing, well, crap. Often a toilet tank doesn't fill fully or overflows. Quite frustrating this is. Here is a very simple and well known technique to fix this.
Step 1: Remove the Tank Lid.
Having to push the lever more than 90 degrees and doing so repeatedly just to have a good flush is a bad sign. First thing is to remove the heavy lid and setting it aside carefully.
It's all too apparent the water level stain for what was optimal water level. Some fool had tampered with the toilet tank float! I pity the fool who tempers with toilets. Arghh!
To adjust the fill level is quite easy. The stop screw has a backing nut that needs to be loosened. The screw need to screwed away from the water valve or towards the float.
Just monitor the filling up to make sure it locks off at the right level.
If the tank was overflowing then a possible solution is to carry the screw towards the water valve. Hopefully the valve is good.
There will be varying tank valve designs but most float valves have similar adjustment hardware.
Step 3: Happy Flushing!
Now with the water level at the right mark, a simple touch of the level and boom. Bye bye crap!