Glass tends to break due to the glass getting too hot, so lowering the LPI to 200-250 is ideal. We recommend defocusing especially when the tube is so powerful. By lowering the bed about 2mm, it will spread the beam out over a larger area, making the spot size slightly bigger and the heat less concentrated.
Even trying a 4" lens will help naturally increase the diameter spot. Glass has different thermal properties depending on how much lead is in them. More lead means it will heat up quicker and hold heat longer than a glass with less lead. This is why crystal, a really nice glass, is not ideal for lasering. You get the best results with the cheap glass you can buy at the Dollar Store! Try to get cheap glass with a low lead content.
Start with your power as low as possible 20-35% and keep the speed relatively quick 300-400mm/sec. Keep the LPI as spaced out as possible 200-250 and defocus by 1-2 mm. Additional ways to help dissipate heat include: applying a wet piece of newspaper over the glass, dish soap, or using a 4" lens.
Book time with a qualified technician and get help with Glass Engraving.