There is an elaborate equation, but it boils down to this. Every 50 calories equals 1 WW point. For higher fat foods, you add additional points. For higher fiber foods, you subtract points. So, 21 points a day is roughly 1,050 calories.
The secret to making points work is to follow the plan and not try to restrict them to achieve a goal faster. They're balanced for your current weight and activity level.
I lost nearly 30 pounds in 6 months by sticking to the plan. I'm a T1 on a pump as well