Welcome to the Arduino world.
What you asked about should be very possible. First would need a little information. What is reading the angle, a pot generating a analog voltage read by an analog input pin? Is so you could write your sketch and create a int variable called cal that would be initilized to zero at the start. If at any time you needed to 'adjust' your reading you could have two push buttons, one called up and one called down that could increment the cal variable each time you press it. This cal value would be added to the analog input reading, there by adjusting the reading.
There are probably many ways to do it but we would need drawing of your circuit and your attempted code to go further. You could also have a pot that would generate a value as you turned it that could be added or subtracted from the angle value variable.
Welcome again;
Lefty