Fitness Objectives for Wallacei

Hello, i am trying to work my way through Wallacei and currently trying to define some fitness objectives… having followed the tutorials and primers ive not had much success in terms of where to start. I’m even beginning to think that it is simpler than what i am making out… (ive even tried the sample fitness onjectives but they are no use to what i want to do.)

Can anyone offer some support?

Ideally i am trying to maximise / minimise the external surface area.
Define a minimum / maximum cost per m2 of footprint

and generally anything else than can help wallacei give variants…

Ive used packrat to bin pack a series of volumes from a spreadsheet into a building facility.

Any help would be appreciated in terms of how creating fitness objective can actually be achieved.