how to build a reliable engine

Head gasket thickness is dictated by block deck height and if the cylinder head has a squish or quench pad then piston to cylinder head Clearence, type of con rod also has to be allowed for and max engine rpm too, combustion chamber volume and piston dish ( convex or concave) has more to do with compression ratio than h/g thickness to a point.

all very true dan but lets face it most on here are gonna use stock rods, and nearlly all the pistons are concave so if you use a 8:5.1 piston you can pretty much work out what size of gasket youll be needing for the average 400 odd hp build
plus as far as im aware cosworth only do 4 thickness gaskets to choose from

johnny....just because your running excess of 400hp does not mean you automatically need a 2mm h/g.
by dropping the compression ratio youll make it safer to run a higher boost but dependant upon other factors you could end up making it laggy and dead off boost if your using a big tubby

and ive just wrote all this after a night out down the pub lol