The target ranges for type 1 diabetics are to be 4.5 - 7.5 before meals. So, yes 20.3 is too high. However, if you are recently diagnosed there is an element of gradually stepping you down to the target levels because your body will be used to the high levels and suddenly running at healthy blood sugar levels may feel too low for your body.
As a newly diagnosed type 1 you should be in close contact with your diabetic specialist nurse for advice on blood sugar levels and insulin doses. If you're concerned you're levels are too high, you should get in touch with your DSN.
Have you been given any guidance on correction doses? Have you been advised to test for ketones when blood sugars are high? Do you have a ketone testing kit?