mulleye,
i believe you mean ECON speed? LRC does not change with wind. it varies with weight and altitude but not wind. thats why before the 'fuel crisis', most airlines i know simply flys a CI which is close to LRC at OPT ALT (and caters for wind at the same time). however, in the current situation, most of them now flys a much lower than LRC speed. as for rule of thumb, i guess the QRH (boeing) would be a good start?
SR