Sorry about the thread necromancy, but I figure some people might want to know the solution to this mystery. Yes, the ending you get is entirely dependent on your shooting skills, specifically your accuracy. The two-byte block at $07D6 tracks the number of bullets you fire and the two-byte block at $07D8 tracks the number of times that enemies have been hit by bullets. At the end of the game (I assume, I don't know nearly enough about hacking to confirm that this is the case), $07D8 is divided by $07D6 and the result multiplied by 100 to give an accuracy percentage. The values given by Pennywise are threshold percentages; 50% or greater accuracy gives you the best ending, 30% or greater accuracy gives you the next-best ending, and so on.