Hi Michael.
Not answering your question directly, but it is far easier to make a dew strap with nichrome wire rather than a ladder of resistors.
I have made many in less than 10 minutes and the wire is cheap too.
For calculations and explanation (including a great excel worksheet) see
http://www.blackwaterskies.co.uk/201...ew-heater.html
Sorry if this isn't answering your original question, just hope it helps a bit?
Stu