How I calculate initial point cost is basically a combination of gut and comparison as you guys both suggest.
First, I use the gut feeling utilizing only the original 10 Hasbro cards as a scale. Basically, which of the original cards is closest to the custom.
Then, like Streaky, I look at all of the HelmAverse cards in that cost range and eyeball where the new custom would fit in line.
After some playtesting at around that point, I tweak the points as warranted.
Last, when I settle on the point cost, I once again review the HelmAverse cards at that exact cost and ask myself, "how often would I draft this new card instead of one of the existing cards?" I do this because if the answer is "never", then the card is either overcosted or just too boring and may need to be scrapped.
More recently, I have been also utilizing
this calculator offered by Giantslayer as a double check for my initial gut feeling. at the very least, it gives you a minimum for the base stats.
Without going through all of these steps, my initial gut would put Starfire at about 200 and Blackfire at 180. Totally just guesstimates at this time.
As for playtesting, I'm usually fine with a word doc that I print up with the stats color coded. I don't need a card mock up but I'll admit it's always fun to have one.