Three marks at two ten means three bogies at 210 deg...

Exactly. Why would anyone think any different?

Three marks at two ten means three bogies at 210 deg...

How do we know Rebels don't use radians?

I think it would be even worse if they used angular mils. "Three marks at thirty-seven-thirty-three!"

Well, we know the Empire is British, and Britain uses the metric system, so they must use grads.