Homebrew PCBs

Does anybody actually know what the required (or recommended) adhesion 
temperature for typical negative acting dry film is?

I for some reason thought it was around 70 to 80 degrees, but I've been 
experimenting with much lower temperatures (maybe 55 degrees C at the 
most) with very good results (I'm using a clothes iron to laminate it 
and the temperature is measured on the hot plate as best my thermocouple 
will).

So I'm wondering what the actual necessary temperature is and how cold 
can we go before adhesion is compromised, for the clothes iron method 
the colder temperature we can use, the better, because of the danger of 
blistering the polymer with irons that are too hot.