I set mine so it gives absolute pressure. Then I only have to enter in the pressure reading into by ballistic program and the altitude setting on the ballistic program can be ignored.
You must adjust your altitude setting on the 3500 though for this to work properly. You do that by getting the correct altitude number from a map, GPS or other device then change the altitude setting to show the correct number. Then the absolute pressure reading will show correctly.
To adjust the altitude press both left and right buttons on the 3500 at the same time then you arrow left or right to make the necessary change.
HTH,
Scot E.