Toilet flush only uses about a fourth of the tank before flapper shuts. I have to continue to hold the handle down to flush completely. If this is done the toilet flushes with no problem. I have replaced the flapper and adjusted the chain to both short and long but nothing helps. I need to resolve this before it makes me crazy.

This is a lumbing question not a forum help but I will answer after a question or two. When the tank is full does it reach within 1?2" of the top of the overflow tube?

It reaches about 2.5 inches from the top of the tube.

... then Ballengerb1 nailed your problem down... ;)

You need to set the water level so it is approximately 1/2" to 1" maximum below the top of the overflow tube. To adjust the water level in the tank requires that you make an adjustment to the toilet FILL VALVE (or specifically the float mechanism of the fill valve).

YouTube has some great videos on adjusting water levels on different fill valve assemblies.

Let us know how you make out, OK?

Mark