To perform a clean 90° angle, just use a v-bit and make a channel along the bend line, as you would do for ACP. It will bend easier and will give you less corner radius. For better strength, you can leave a 1..1.2 mm for bending, but it's not necessary since you can also insert some Super Glue in the channel, after you bend the polycarbonate.
This way you will also have a perfect bend line and the sheet will not curl because of the heat.