Important to know when the boat goes faster strait off the wind. It is different for all boats.
My Hydra goes faster strait off the wind until 8 knotts of wind when ity starts to pay to tack.
This is with only main and jib so with a spinny the formula might be different but some timed runs in a steady breaze will let you know and after that you just have to define the critical wind speed when the change occurs.