Save your money. The iPhone apps are cheap and precise enough for leveling a pinball table. Suggested angle settings are just a recommended starting point and personal taste. You'll always finish adjusting the game by feel anyway.

Even if you want to follow someone's setting of the same game, your game has unique flippers, rubber band, playfield that will affect gameplay in its own way. In fact, you may even need to re-adjust the same game as time goes by. You may need to increase or decrease the angle to compensate for weaker flippers, old rubber band, non waxed playfield, etc. A extremely precise inclinometer won't tell you that...

I'd use a more precise tool only if I was moving locations and wanted to keep the same gameplay feel.