I have purchased a flexible stem screwdriver sold to replce GM headlights. It only came with Torx head tips but was able to use a small replacement straight screwdriver attachment in it (like those used in drills) In using this, I never thought my turns out were accurate on all 4 carbs.
My favorite however is just a small straight pocket sized screwdriver in which I custom ground down the handle to fit in between the carbs base of the engine. I then took a felt pen & marked the handle with quarter turn marks which I use for a referance.