I use a metabo chasing machine that cuts a chase 25mm wide and 25 mm deep , then remove the middle with a chisel in the multi drill , then install cables in 20mm round conduit with conduit just into the 20mm holes in metal boxes , plaster with one coat plaster and reskim and smooth of with trowel and water spray , , this leaves the conduit 5mm below surface , when I'm finished its ready for decoration ...
I find that anything less than 5mm may cause the plaster to crack if too thin in the future..