well thanks guys! the kind and supportive responses have put my paranoia in check so I guess I'll stick to this design for now! (but I do have another in mind with a pretty unstable medium in mind, more of that later..?)
in fact I made a few more of the blast markers but I shan't bother to take a picture as they're all much of a muchness. if you've seen one debris cloud done like this you've seen the rest really... I've kept them consistent colour/form wise, not much difference in them except for the randomness of the shapes...
BUT! I had an idea for a nova cannon template:

inspired by the pressed-tin 40k vortex grenade templates (or those realm of magic ones for whf that I use as planets) this is made of the lid from the sainsbury's mens antiperspirant. (classy I know...) Its a handily sized 5cm accross, simply cut down to be more of a perfect half dome, added 1cm hole in the top and decoupaged with watered tacky glue and standard grade toilet tissue.* with a few creases and swirls deliberately left in for texture. simple paint of white, and dapple on yellow, orange, red and then black with a sponge (working out from the centre) and its job done.
yes I know , in game, I could simply use the bearing compass I made (and probably still would do) but again, this was a hobby challenge and I wanted to see if I could make something more visually dynamic. I think I succeeded... but, again, I'll leave judgement up to you...
*note
I originally attempted to cover this with the clump coarse turf, as to match the blast markers, but it ended up a disastrous result. this, I feel looks visually fitting for an awesome sized explosion that rends the very fabric of space, yet somewhat practical in terms of potential gaming use...
the picture is a bit crappy and is not doing it the best justice but its raining here so this was taken on the window sill. when the weather improves here I'll endeavour to get a better shot with some ships and the other markers (because that was fun last time!) Hell, I might even completely re do this again before then!