The usual conversion factor is one-third. That is, to convert from compressed (cake) yeast to instant dry yeast (IDY), you divide the weight of the compressed yeast by one-third. For active dry yeast (ADY), the usual conversion factor is one-half of the weight of the compressed yeast. So, 1% compressed yeast becomes 0.5% for ADY and 0.33% for IDY. Remember, these are conversions by weight, not volume. The following conversion chart may also be helpful: http://www.theartisan.net/MainCommFrm.htm
(click on Yeast Conversions on the left panel).
A good primer on yeast can be found here: http://www.pmq.com/cgi-bin/tt/index.cgi/noframes/read/24138