One of the fields available from isobaric grids from any model, including the RUC, is height of the constant pressure surface. That is the best way to relate isobaric levels to actual altitude.

If you want to obtain flight level altitude assuming altimeter setting of 29.92 in, the relationship between pressure and height is fixed through the U.S. Standard Atmosphere (1976). You can find information about this on the web from doing a web search. One site I just found by doing so is http://www.pdas.com/coesa.htm.

Stan Benjamin