Firstly, cut down the MAXRaft perimeter profiles to the correct height and width for your rebate prior to installing the steel. This will allow you to obtain a nice, clean cut of the polystyrene making it easy to fit the rebate.

Secondly, while you can use the MAXRaft perimeter profiles as a base height for the rebate, always check the final rebate with a laser level for height and off the profiles/bracing for the horizontal dimension into the slab. This will ensure that the rebate is not affected by slight undulations in the ground causing the perimeter profiles to rise/fall from the actual finished floor level.