Paul Burridge wrote:

...John's formulae didn't appear on his
post for some reason,

I hate it when I find a page and forget to paste the link:
http://www.qsl.net/wa7zcz/area2/page34.html
from (remember this old book of electronic data?):
http://www.qsl.net/wa7zcz/area2/t_of_c.html
--
John Popelish