If you leave the ports at their defaults, then you don’t need to specify the ports, just use http://IP or https://IP (http will redirect you to the https port). Otherwise you’ll need to specify http or https and the correct port. ie https://IP:PORT
When you say Zeroshell’s admin address of 192.168.0.75, there’s nothing special about that address other than it’s the default IP for it’s LAN interface. If you change it’s LAN IP, then you’ll need to use that new IP or any one of it’s other interfaces’ IP address to connect to the admin page, unless you tell Zeroshell to switch to safe mode at the local console, then it defaults back to 192.168.0.75.