http://atsb.gov.au/pdfs/carb_icing.pdf
quick googling for a pic.
Just to show you graphically that 0deg C isn't bottom boundary and it's also about 'visible moisture' or relative humidity if wanting to be nit-picking.
If you have high humidity, you could still get carb icing above 28C.
Pilot's Handbook of A. Knowledge mentions 20-70F which works out as -7 to 21C.
That'll be the main range to watch out for. I think the dew point spread you mention would be applicable to situations with higher temp but also humidity.