Your theory seems sound to me - you should try it and find out. Since that's a rather boring answer, I've also converted your pseudocode to real code:
<cfset pdfScale = 100 />
<cfset pdfObj = "" />
<cfdocument format="pdf" name="pdfObj" scale="#pdfScale#">document contents</cfdocument>
<cfpdf action="getInfo" source="#pdfObj#" name="pdfInfo" />
<cfloop condition = "pdfInfo.TotalPages gt 1">
<cfset pdfScale -= 5 />
<cfdocument format="pdf" name="pdfObj" scale="#pdfScale#">document contents</cfdocument>
<cfpdf action="getInfo" source="#pdfObj#" name="pdfInfo" />
</cfloop>
Depending on your setup, you might also want to abstract the creation of the PDF into a function, so that you don't have to re-write all of the content twice on the page. Or you could use an include. Heck, if there is any sort of complex processing going on to render the HTML for the PDF (which I assume there is, since you're making a calendar), then you might even want to pre-render the content and re-use it, like so:
<cfsavecontent variable="docContents">document contents go here</cfsavecontent>
<cfset pdfScale = 100 />
<cfset pdfObj = "" />
<cfdocument format="pdf" name="pdfObj" scale="#pdfScale#"><cfoutput>#docContents#</cfoutput></cfdocument>
<cfpdf action="getInfo" source="#pdfObj#" name="pdfInfo" />
<cfloop condition = "pdfInfo.TotalPages gt 1">
<cfset pdfScale -= 5 />
<cfdocument format="pdf" name="pdfObj" scale="#pdfScale#"><cfoutput>#docContents#</cfoutput></cfdocument>
<cfpdf action="getInfo" source="pdfObj" name="pdfInfo" />
</cfloop>