According to this book, the chapter on combustion http://www.datafan.com/Horsepower_Blog/gordon-jennings-two-stroke-tuners-handbook/
The squish band should be the exact taper of the piston, cover about 50% of the area of the bore, and be between .015" and .060" from the piston at TDC.
With that being said, I've put slot of top ends on 2 strokes, but I'm not an experienced engine builder. Read the book I sent you. It was written in 1973, but I believe this book is why we have the high performance liquid cooled quadracers that we have.