What rpm do the markings on the dash mean? I was never really sure.

 

Each line is an increment of 250 rpms. So the first white line above the 0 on the tach is 500 rpms.

 

Check the rpm with a real tach, that you or your mechanic should have. The stock tach on the truck, is not very accurate.
I have mine set at 850, in gear and air on.
A word of advice, with age and wear, the top of the set screw, may develope a groove in it, this will affect, your adjustment, inspect this closely.
I was surprised, at how little it takes to raise the rpm by 100 rpms.