Creating RF Link Budget using Visio

Hi

I am trying to use Visio to create a RF link budget, but I am clueless how to do it with the coding and formula. Basically is to sum all the value of the connected shape/line values and reflect at the end.

An illustration of the outcome is shown below: rectangle has a value of +5 and the line has a value of -3. hence, the triangle able to reflect the sum of the connected shapes, +2.

Is there a simple template or coding that will automatically sum all the connected path values and show it at the end?

regards

Ben