The Stearman Erection Manual says to put two 1/16 in washers under each mounting point of the horizontal tail, two under the front and two under the back on each side. Then it says that if this doesn’t work out properly, then you can have as many as eight washers under any mounting point. That sure isn’t a lot of help!
Before a test flight to check this incidence setting, find out where the elevator trim handle is positioned so that the trim tabs line up directly with the elevators. (The neutral position). Put a small piece of white tape at this position next to the handle. It should be straight up.
Load the plane the way you want it for the best trim condition. (one or two people? Amount of fuel? Luggage compartment?)
Then in flight, trim it out to straight and level at cruise rpm. See where the trim handle is. If it is forward of the neutral tape mark, then you need more washers under the front mounting points (or less in the rear). If it is back of the marker, you need more washers under the rear mounting points (or less in the front). In flight put another piece of tape marking the handle position at straight and level. Then on the ground you can position the handle at that point and see how much deflection you had to have in the trim tab.
The goal is to get the trim tabs at neutral position at straight and level cruise, and average load.
How much off-neutral is too much depends on how picky you are. An inch off could probably use an adjustment. Don’t change more that one or two washers before another test flight. The tail brace wires have to be loosened and then re-tensioned after adjusting the number of washers. So each setting and test flight takes some time and effort. Err on the side of having a little down trim at cruise.
If you end up with more that two washers at an attach point, put tape around them and form a small cylinder. Then position them in place as you move the bolt into position.. Otherwise you’ll be chasing a lot washers on the bottom of the fuselage.