I am installing a motor setup in my 3mtr Pilatus B4 any guidlines as to how to ascertain a good thrustline?

You'll need to use an incidence gauge to find the chord line of the wing. Note that if this model uses a flat bottom airfoil that the flat bottom is NOT the chord line.

With the chord line angle determined you want the motor to have roughly 2 to 3 degrees of downthrust and about 2 degres of right thrust. The amount of downthrust is tied partly to the design and partly to how far back you are comfortable with placing the CG location. The less stable the model the more efficient the glide will be. And along with this comes a lesser need for downthrust. If you are comfortable flying a model which is stable but requires more pitch management than a normal trainer or advanced trainer then you can use closer to 2 to 2.5 degrees of downthrust. If you prefer a more trainer like amount of pitch stability which strongly returns the model to straight and level then you'll want more like 3 to 4 degrees of downthrust. And likely a touch more right thrust as well.

Mr BMatthews, I thank you very much, Cheers