http://forum.liberty.asn.au/viewtopic.php?f=10&t=5714&start=15You'll never come close to a "colour" match unless you re-tint the rear glass as well.
You'll also not be able to get close to "darkness match" without being illegal.
So, look at the above thread and see my attempts with a tint specialist mate to get as close as possible to a match colour and darkness.
Regardless, just decide - I want "LEGAL" or I want a "match". The two aims are mutually exclusive!
EDIT - To work out a transmission percentage you with multiple tints you would have , say 20% with 35% on top, would be 20% less 35% of the available light is .35 multiplied by .2 being .07 or 7% transmission.