The measure example does NOT meet the specified customer requirement - that the ratio cell where Internet Promo sales are blank should read "NA" rather than blank. This in turn causes an issue with the Calculated Measure, since it contains text data and can not be formatted as a % as specified.

I'm also not sure that "ISBLANK(sum(...SalesAmount))" makes sense - it implies that the Sum of 1 or more BLANKs is BLANK, and I'm not sure that's actually part of the spec (although it does seem to work!).