Any way to use a custom prop in an equation?

Hello everyone

Here's the background for my question. I often need to model drive
chains (roller chain) for the machinery we design. The shape is often
composed of several lines and arcs. To get the length (parametric) in the
BOM I used to put a dimension on each segment, create a construction line
and make it equal to the sum of all the segements. Then create a custom
property named Length and make it equal the the length of the construction
line. This is of course a lot of work.

The simplest way would be the following: when you select all the lines
at once and use the measure tool, you get the total length. However it
doesn't seem possible to pull out this number and make a custom property
with it.

So here's what I thought we could do. If I have a config that "cuts out"
all but 1 inch of the chain and find the weigth of it, I could go back in
the other config and divide the weight by this number to get the total
length of the chain. However, I am not able to put custom properties such as
mass in the equations (using SW 2005).

So until someone has a better idea, I have to create 2 configs (one with
a one inch chain) and use a design table to fill in the custom props using
equations.

Thanks in advance for any ideas

Pascal Dufour