The thicker the glass (within reason) it is easier. Drilling 3/8" or 1/2" glass is easy and with a diamond bit it's a piece of cake, go slow and lot's of water. Just be carefiul on the "push through" at the end or you will get chips taken out of the glass. Some like to clamp a piece of wood on the back so the bit doesn't "punch through" quickly.
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-=Bryan=-
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