You can pick up a cheap inductive tach on ebay for about $10.00. Use it to set your idle speed and put in your tool box for next time.
That will work for you.Thanks for the suggestions, guys. I went the cheapo eBay/Amazon route and picked one up for about $12. It's one of those that has a lead to wrap around the spark plug wire and measure RPM's that way.
Hopefully it'll be precise enough to get an accurate measurement.