If you are looking at the port side, find the stateroom with the bed by the bath. You can find it by locating the "square" which annotates a stateroom with fold out sofa bed. All those staterooms have bed by the bath. Once you find the bed by the bath, the stateroom to the left will have bed by the balcony and will share the same "alcove" as the staterooms are built in modules of two.
Your example of 1283/1285, to find the stateroom with bed by the bath, you count from 1287 (square/bed by bath). Every other stateroom will have bed by bath, so 1283 will have bed by the bath. It will share an alcove with 1285 (bed by the balcony).
I assume from your post you are not looking for "connecting" cabins, just alcove sharing. Connecting staterooms will have a line connecting the two. Example: 1251/1253. Those are the only two staterooms in your area that are connecting.