Right now, if you make a custom fx, "angle": -1 allows you to set the angle of the effect. This same interaction could be used to set distance for some fx. Particle travel distance can be calculated with lifespan and speed (ignoring the random factors); lifespan or speed could then be scaled to the appropriate amount for the particles to reach the distanced specified. Perhaps the distance could even be provided as a parameter to /fx. This would be useful in many common cases. First and most obviously, variable-size explosions, cone attacks, beams, and the like. Much easier and more intuitive to use. And, usually, you have a specific range in mind for that sort of effect. Second, with /fx targeting from one token to another, an fx could be used that ends exactly at the second token. Think like a Magic Missile or a Ray of Light or whatever; anything that wouldn't keep going past the target.
