I have just been modifying a piece of equipment that is housed in 'furnace board', which is a thermal fibre re-inforced cement product for high temperature applications. The fixings are 6 mm helicoils about 9mm long, and the originals are set to be flush with the outer surface (ie the side from which the strain is coming).
Having to put in some more helicoils into a 25mm thick board I came to the conclusion that for maximum strength they should be set in from the outer face to allow a larger 'cone' of this rather brittle material to take the stress, so I set them back a further 6 mm. Was I deluding myself or do you reckon it'll help?
AWEM