ndchristie who taught you to measure perspective like this? Or what you are measuring? It simply makes no sense to me! My own perspective knowledge isn't very extensive and I am not very formally trained, but I can do multi-point perspective and even curved points and whatnot, and this is quite unlike anything I've ever seen anyone slap on a piece to explain perspective and depth. Are you improvising?
Arachne, you're doing a very ambitious project here. There are many concerns. One of them is the effect of mist (if you're going for mist). I suggest you look at a lot of misty moors and lakes on gi and see how the mist mainly homogenizes everything in the picture, making the perspective seemingly break and so on. You NEED this effect of 'is it far away or is it near?' that the mist gives you because it helps the drama of the subject matter.
Right now you have a very high shot of the action, and a high shot (since we're human beings and we're used to look at he horison from 2 meters high at most) dehumanizes the situation. We become the silent observer, uninterested in the terror that is going on. Is this what you want? I don't think so. I suggest you go with an actual human point of view, if not even toads' view (I didn't mock up toads view it requires extensive third point perspective reworking) to emphasize the powerlessness against such a creature.
Also, in Lovecraftian fashion, I will suggest that you show much less of the creature and much less detail. Let the mind imagine, that's the rule of cosmic horror. If it is CLOSE, it is more defined and darker, if it is FAR (and such a creature would have to be) then it is coated more in mist.
Don't scribble when you are setting up a construction of a picture. Every superfluous line muddles up the concept. Place firm geometric shapes and figure out their coorelations before you attempt any rendering.