To get the correct URL, use the EditURL method that is available in PortalModuleBase and is inherited by all modules.  There are many different overloads for this method depending on whether you need to pass parameters to you edit screen and whether you have just one edit screen or multiple screens and want to differentiate between them.

Also, the best place to look for answers like this is to see how the standard DNN modules address this requirement.  Since the code is readily available, and works, it should be easy enough to find an example that is close to what you are trying to achieve and then just do something similar.