How to caulk uneven gap in corner of kitchen backsplash?
I think the trouble is with your tooling method. It's wobbly. :)

Either trim your tube better to act as a forming tool itself, or make a tool out of cardboard or plastic, or purchase one of those found at hardware stores to achieve more consistency. 

You could also mask the lines with tape, caulk, and remove the tape. This may leave raised edges, though.