Short cage derailleur: how to find out maximum sprocket differences? I'm building a kind of new bike and would like to put a triple or compact crankset on it. 

I've still got this NOS huret derailleur set (front, rear and shifters) laying around which would match the bikes flair pretty well; but I'm worried it won't fit.

How can I calculate the maximum size difference in front and rear sprockets this derailed can handle (by looking at it's dimensions).

The derailleur model is the Huret Svelto. I'm looking into buying a crank with 34t and 48t sprockets and have a 7 speed rear cassette of 13t - 24t (but these are loose sprockets and can be changed).

What is your advice?
As a general rule a Short cage derailers should only be used with a single front ring, medium cage with double front rings and a long cage for tripple. 

Naterally this can be effected by the amount of travel your bike has at the back.  If it is a full suss then as the rear wheel moves through it's travel the chain line will get longer, placeing more tension on the rear derailer.