I Have Severe COPD. I Have Been On Oxygen For

I have Severe COPD. I have been on oxygen for 16 years. I also have heart desease and had CABG X 4 10 years ago. Recently I had an exaserbation due to allergies causing a lung infection. I got antibiotics for the infection but had difficulty getting my stat about 80. I have always been on 2.5 levels on the concentrator but when I am moving about or exercising, I move it to 3. My question is how to I know when I need to move it higher. I can check my stat regularly. Is there a number where it should be for me to know the level on the concentrator is working well.