StoneCold:
If you or your organization don't have a physical properties database available, then I recommend you read up on "The Properties of Gases and Liquids"; Reid and Sherwood; McGraw-Hill. There, in the Viscosity chapter are the recommended methods for handling this problem.
This book is one of the necessary and prerequisite tomes required by all Chemical Engineers - especially those involved in process design. In my opinion, you should own this book long before you invest in a Perry's Handbook.
I hope this helps.
Art Montemayor
Spring, TX