You can consider that there is about 10 inches between the contact point at the bridge and the string ball. More or less depending on the bass.
You should be able to make your calculations with that.
I advise that you put a piece of wood to decrease the string angle, where the string will exit from the body though. Try to get the same angle a regular tailpiece would give on an acoustic bass.
Of course this is only applicable if you want to use a bridge with a relatively normal height.
Look at this for an example of what I mean:
http://www.readcustom.com/eub5close.jpg
