unless the edge on your metal scraper is properly sharp i would reccomend using a razor blade/stanley knife as you are more likely to rip it out with a blunt scraper, rather than take off the excess.
heating up the base with your waxing iron around the gash is good as the ptex will be less likely to crack when it cools and small amount of thermal expansion on the base will give a better grip once it all cools down.
I would also reccomend shaping your gash to give it a bit of an overhang on the edges of it, it will hold the ptext better. also roughening up the suface inside the gash with a couple of lines with your razors blade will improve hold.