So, you want to create a new dimension to match an existing one, but you don't know what the DIMSCALE of the existing dimension was when it was created?  Simplest way to find out the DIMSCALE of an existing dimension is to double-click the dimension to open the Properties palette.  In the palette expand the Fit area and look for the listing that says "Dim scale overall".  Now you can change the DIMSCALE to match!