As of iDevAffiliate 9, the following information is obsolete and no longer needed. iDevAffiliate will automatically detect the correct path whether you’re using a subdomain or not. There is no need to edit any files. Just complete your installation in to your chosen subdomain and iDevAffiliate will do all the work.
To use a sub-domain instead of a folder just simply create the sub-domain and upload the files like normal. Then all you have to do is edit the /API/install_folder.php file and use no directory name at all. You will only have an entry in this setting if you’re using a directory in which case it would match the actual directory name you’re using. Most people don’t use a folder at all when using a sub-domain.