How to board a chimney breast?

I've hacked my chimney back to brick and will be dotting and dabbing plasterboard onto it.

When doing a "side" wall, do you overhang the front part of the plasterboard by approx 25mm so that the board going on the face of the wall will butt up to it? or do you just cut them so that the edges of the board are flush with each other (although once dabbed, will be a 20mm gap or so) and just fill in the gaps with board adhesive prior to putting beading on?

 

why 25mm? 10mm overhang will be enough to allow for the adhesive.
trim it up with a surform at the end and fill with adhesive if you like before beading.