Tube amp bias adjustment advice

You do not measure from the back of the speaker to the wall behind it. You measure from the front of the speaker to your nose at the sitting position. 

My speakers are 20 inches and 23 inches from the back of the speaker to the back wall but if you measure from the front of the speakers to the sitting position they are both exactly 84 inches. Measuring from the back of the speaker to the back wall is a good starting point but if you want to be exact you must measure from the front of the speaker to the sitting position I’ve been doing this for over 50 years and I have yet to have the same measurements from the back of the wall to the back of the speaker there’s always some adjustments needed I hope this clarifies my position. Warmest regards