I have a small 'line level' that simply hooks into the center of the
chalk line. With one end firmly attached, the other end of the line
can be raised or lowered incrementally until level. Only takes
minutes, seems perfectly accurate and IIRC it cost about $5 or so at
local lumber yard, hardware store or Ace, maybe. First time I saw one
being used was a siding installation crew. Ought to work for you.

Over any distance at all, the line level weighs enough to sag the line
substantially -- which means that it won't give an accurate reading unless you
have it in the *exact center* of the line.