as a mug amateur who has done a couple of shed slabs, I would

use the reo, for the small extra cost it might save you a lot of heartache down the track.

look at rebating the edges for water resistance. Just be accurate with your measurements. I have also sat my own shed on strips of foam concrete expansion joint. Does the trick nicely.

Are you barrowing up hill, down hill or on the flat? Factor that in to your calculations.

Don't be a hero with a chock-a-block barrow, take what you can handle comfortably. Watch for bumps as they will cause slops.

Get a few mates around to help barrowing. Having more help than barrows is okay becuase you can tag team on the barrow while others shovel/level/screed.

No beer till the job is finished. Very important.

Jon