Share of current Excel users who can only open .xls files?

We’re creating a tool in Excel, using the .xlsx format, that we would like to provide on a broad basis as an open source document. Ideally, this spreadsheet should be usable to all Excel users, but we’re realizing that saving down to .xls removes some key features. I’m trying to find some estimates for the percentage of Excel users who are on older versions of the program and only able to open .xls files. I believe there are workarounds to open .xlsx files — converters, Google Docs — but for the sake of justifying this decision, I’m mainly curious if anyone has any idea of what the market share is for folks who have older versions of Excel that can only open .xls files.