I would like to know how should I calculate main engine location.

Where can i get information about it on the internet?

Sorry to sound silly, but do you mean location, as in best balance, power etc?

Or
Do you mean alignment of the main engine to the propeller shaft.

ive seen some engines put in some wierd spots. i worked on an older twin engine that had the the stbd. engine mounted 10 ft. aft of the port.

Design a ship, then maximise the cargo carrying capacity of your design, then look for an engine that fits in the small space you have left. Engineering does not dictate where an engine goes, commercial realities do.