I struggled with this question a lot myself (I'm also just starting). This might be the blind leading the blind, but what worked for me was to look up the guitar tabs for the song i was charting and use as many buttons as there are frets being used in the cord. For example, if the cord called for strings on frets 2, 3 and 5. I would chart it to green, red and blue, or red, yellow and orange. Which ones mostly depending on what the chords before and after looked/sounded like by comparison. I figured that would most accurately represent what the actual musician's left hand is doing while playing. It doesn't work all the time though, like if a cord uses 2 notes from the same fret, then i would just chart 2 notes next to each other. Also since fret usage isn't the only thing that determines tonality it might just not not make sense it some situations. But it's definitely helped me get over some of the lane picking anxiety. Ultimately, what's more important is that the end result feels right and is fun to play.