Besides trial and error, or guestimating the ideal tension using a variety of tricks to gauge it there's no easy way.

Here's a trick. Hold the FD outboard a bit so the cable is too tight, and set the pinch bolt to a very light hold. Gently draw the wire away from the down tube sliding it out under the bolt until it's just right, and tighten.

A friend has his own variation on this technique. After setting the tension too tight he shifts to the large chainring and lets the excess tension pull it out. It doesn't work for me, but he has the touch and nails it every time.

As far as I'm concerned, adjusting cables without the luxury of the convenience of an adjusting barrel is a waste of time, so I use inline adjusters of FDs when the frame doesn't have them.