Good question. We’re just multiplying the direction vector by an arbitrarily large value (the direction output of the previous node is a normalized vector, with a length of 1, which isn’t enough to intersect with the plane). The only thing that matters is that this value is big enough to ensure that we do have an intersection with the plane. I put 10000 in this example but you actually need to make sure it is at least the distance from your camera to the projectile origin location.
If you are using a spring arm component for example, it could be the spring arm length x 1.5 (multiplying by 1.5 just to make sure we reach the plane to intersect). Else, you could use the real length of the distance between the camera component and the projectile origin location (again, multiplying by a value > 1 is always a good idea to ensure an intersection).
The line plane intersection node does a simple mathematical check, it doesn’t perform any expensive collision and nothing else of the sort, so having a bigger value than necessary for this multiplier isn’t going to impact performances at all.
Edit: actually, since your game is not a ‘complete’ top down (the camera is at an angle), the above isn’t exactly true. Because the player can potentially click at a ‘grazing’ angle, we need this line distance to be quite a bit bigger than the distance to camera. You should do some tests, in case you notice some intersections are not happening. But I would be you, I would just use a very large value.