how do you convert percent transmittance to mg/l

There is none.

The lower transmittance of film would indicate a higher Optical Density (OD), or absorption, because as concentration increases, % transmittance will decrease. This is because of Beer's Law (A = 2.00 -[log (%T)]). As you can see from the equation, transmittance is directly proportional to the outcome of absorption. High %T = Lower Absorption.
Low %T = Higher Absorption.

A=logIo/I

The percent transmittance grew to steadily higher numbers as the experiment progressed because the light reaction was able to occur. However, the dark cuvettes had stable levels of transmittance because light is necessary to excite electrons, which, in turn, reduces the DPIP.