1. Get an Engineer's Triangular Scale
You need an Engineering Ruler or Engineer's Triangular Scale available at staples $5-26. Make sure it is not an Architect's Scale, because those are measured in fractions. You want the Engineer's scale which has a 10, 20, 30, 40, 50, 60 scale - you want to use the 10 scale (10ths of an inch - finding it is a chore). That is the measurement used by MS Office/MS Publisher/Star Office/Open Office.
This works for Avery forms/labels/businesscards/postcards/whatever. In fact, you can use it for other manufacturers of forms other than Avery such as OfficeMax, Staples, and the like and save money.
Why an Engineering Scale? because everything in Publisher is measured in 10ths of an inch, and Word can be measured in 10ths of an inch if you so desire.
Use the 10 Scale: Remember that large ticks are 1", next largest are 0.5", next largest is 0.1", and in between the small ticks is 0.05". measure carefully. oftentimes on postcards like Avery 8386 the side & top margin will be 1.25" and the vertical gap will be 0.5" and h gap will be 0. you will see odd numbers like 1.25" or for business cards 0.75" many times. As I said before, measure carefully.
In Publisher These settings are found in "File"|"Page Setup" area, especially with the "Change copies per sheet" button in that area. Office 2003 and later has most of these settings: Office 2000 is missing a lot of these and the "change copies per sheet" button. I am still looking over Publisher 2007 to see what it has. Publisher 2007 cannot save to 2003 format (naughty naughty, Microsoft!). but it can save to Publisher 2000 format.