The 1/8" thick tiles are will yield a 3/4" wide arrowhead if you're really good. That's a 6 to 1 width to thickness ratio. Most knappers should shoot for 4 to 1 to be on the safe side.
Clear glass comes in much better sizes and quality but it gets old very quick.
The best and cheapest glass I have found is bottle bottoms from carbonized water. You can find them in various shades of green and blue.