You are better off using icalc with the approximate formula I mentioned in ray/src/cal/cal/trans2.cal to get transmission (tn) from normal transmittance (Tn) and normal reflectance (Rn). The computed index of refraction (n) should be close to 1.5 for standard glass.
The following should work, with an example for 88% transmittance glass. The lines with prompts are what you enter, and the lines that look like $1=… are what you get from icalc:
> icalc trans2.cal
> Tn = 0.88
> Rn = 0.08
> tn
$1=0.956521739
> n
$2=1.51380703
-Greg