,,,, mike, most of the time, i'll scratch the glass, a little a bit, to see if it sounds right. than the buyer can do whatever he/she likes. glass is very hard , you need some 60/80 grit sanding paper, to scratch the playing surface, scratch it left to right , than run striker the other way of the scratch marks.
the gap should be somewhere around .125, (1/8 inch), you have to play around with this measurement. also, keep track of all your calls that you make, write them down, and see what works for you.