Well there is a way to measure it, but it will be close, as close as you can get, without the correct tools and simple too.
WARNING - This is okay for a rough check of COL only - use your common sense please.
Get a piece of paper and a ruler with the correct increments, draw a line up and down (this is where your cartridge rim will line up against).
Then draw a line at 90deg on the right hand side forming a "T" laying on its side, this line will need to be measured carefully to match 2.650" or 67.31mm. You must measure very carefully!
Then at the end of this horizontal line yo