This engine's (Skoda Estelle/120, 70s, RWD, rear mounted engine) head is cast iron, the block is aluminium and I'm checking it's flatness. What would be the correct flatness clearance range? I have seen a few sites, but none says it in a straight and specific way, they all say "use a feeler gauge" and that's it. But, what's the correct numbers? Is it up to 0.5 mm?
Modern engine with steel head gasket would require a maximum of 0.003" or 0.075mm. You can have a more irregular surface if you have an older engine with thicker compressible head gasket. The surfaces should be flat so that they match accurately as the error will grow as the different materials expand at different rates during temperature changes.
For your engine I do not know the correct spec but I would recommend a skim if can fit a 0.5mm feeler under a straight edge - that is way too big and would not hold.
Another thing to think about is if the problem areas would allow pressure to escape or are self contained. Air-cooled engines can get away with much more due to the mating area being that much larger, water cooled engines tend to blow into the water channels where the gasket mating area is thin.