As for everything with diabetes, it varies for everyone. From the book "Diabetic Athlete's Handbook" by Sheri Colberg the amount to reduce your basal rate depends on duration of the exercise and intensity of the exercise. For example if you exercise for 30 minutes at a moderate rate, she suggests a 10-20% reduction rate. For 60 minutes of moderate activity, she suggests 20-40% reduction.
For me, if my blood sugar is at my goal (120 or less), I decrease my basal during exercise by 20%. If I'm high starting out, I will not decrease my basal at all. It also depends on the weather. If it's hot out and I run outside, I may actually increase my basal or give myself a small bolus before I run (my blood sugar tends to go up when it's hot outside...don't ask me why). Honestly, having a CGM is incredibly helpful because you watch the trend and adjust accordingly.
To start, maybe try decreasing your basal by 10-20% for the amount of time you exercise. Also, check out the book that I mentioned above and "Think like a pancreas" by Gary Scheiner. Both are good resources for exercising, though Scheiner's book is more than just exercise.
I hope that helps!