The link provided above provides target BG.
Have you also read
https://www.diabetes.co.uk/forum/threads/basic-information-for-newly-diagnosed-diabetics.17088/
This will give you some ideas about why you are testing?
With regard to exercise, testing will be done primarily to understand the impact of the exercise on your BG.
It is common to find that exercise reduces your BG. However, some types of exercise may increase BG at the time (but lower it later) and some people find they struggle to exercise when their BG is too high to start with.
This is all very personal. So it is good to test for you with your exercise.
I recommend testing before exercise, when you have finished and taking the exercise into consideration with your testing over the next 24 to 48 hours.