I am trying to calculate the resultant Tvis value of glazing when the overall value is not provided. The manufacturer has provided specification sheets that show the window pane has a Tvis of 70% but also has a Security Window Film Application that has a Tvis of 86%.
I am currently using the formula: VLT1 x VLT2 = VLT3. The resultant Tvis value is then: 70 x 0.86 = 60.2%.
Is this the correct procedure to calculate the average of two Transmittance Values or for Daylight Calculation purposes is there another method that is stipulated by LEED 2009 to determine the resultant Tvis value.

Thanks in advance for your time and comments