How to report result to the nearest 0.01 mg/L?
Reporting to the nearest quantity means that you report as many decimal places of the quantity as suggested. This is a common analytical chemistry expression given in procedures or analytical protocols. For example, if we say weigh  $\pu{5 g} \; \ce{KOH}$ to the nearest $\pu{0.1 g}$, one would record the weight to one decimal place, e.g., $\pu{5.1 g}$ or $\pu{5.3 g}$, to one decimal place but you will not report $\pu{5.1176 g} \; \ce{KOH}$ even if your balance is capable of doing so. 

Your test report should include at least two decimal places, the report should appear as $\pu{7.24 mg/L}$ or something like that number.