Weight Percent of Solutions Given Without Units
I have assumed this to mean: (mass of solute) / (mass of solution) * 100


Most likely you are correct in this assumption. Percentage compositions of ionic solutes in water are nearly always implicitly expressed in percent-by-weight ("% w/w").

$6\%$ bleach is relatively dilute and will have a density very near to $1\ \frac{\mathrm{g}}{\mathrm{mL}}$.  So, $1\ \mathrm{L} \approx 1\ \mathrm{kg\ solution}$, and thus one can expect to have approximately $60\ \mathrm{g}\ \ce{NaClO}$ per liter of such a solution.