I have an older Kohler toilet and lately it is taking as long as 20 minutes for the tank to refill after flushing. There is a original Fluidmaster 400A fill valve in the tank. I purchased a new Fluidmaster 400A, but even when totally extended it is about 1 1/2" shorter than the old one. Because of this I cannot position the critical level mark on the new valve so it is 1" higher than the top of the overflow tube as the instructions state it must be.

I thought this would be and easy fix. What do I need to do?

Thanks.