A (Robart?) incident meter is a bubble level that is also graduated in degrees and specifically designed for model making. Remember, the firewall will dictate what the (accidental or intentional) thrust line is. In other words, if your firewall is installed with unintentional offset (crooked, eh?) then the aircraft will be diffcult to control (watch the takeoffs). Hopefully I have not completely confused you. Harry Higgly makes a good book on this and well worth reading. (RCM Magazine). To sum up, square (level) the firewall, jig the model on this and set the wing incidence.