How to select source termination resistors by looking at signal?

If I have two devices that some distance apart on a fairly large PCB (could be 1 in, or it could be 20 in). If I arbitrary select a value for R1, lets say 33 ohms in this example and if my digital signal is distorted due to reflection, how can I determine if I should increase or decrease my resistance to get my digital signal correct ? What indicators when I look on a scope, should be key characteristics that my voltage divider ratio is too high or too low (referring to source impedance + R1 and Zo)

Assume that the characteristic impedance (ADD: of the trace) is (ADD: constant but) not controlled (ADD: / known).

^{simulate this circuit – Schematic created using CircuitLab}