Although you do want a high tooth count for a higher-quality cut, an ultra-high tooth count can be detrimental. With a 100T 7-1/4" blade, you'll have to cut more slowly and may burn the plastic.

If using a 7-1/4" circular saw, use a high-quality 50-60 tooth blade, which will perform similarly to the 10" 80T blade that Aloysius Defenestrate's local plastics company recommended.

To help protect against chipout at the edges, you can either tape the cut line or you can sandwich your Plexiglas between two thin pieces of sacrificial plywood or MDF.