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I trust my father-in-law, he knows what he’s doing.

That should be fine in this case, since he worked for the supplying company, meaning he has specific applicable knowledge about how it will be with these elements here. Basically, a 2 cm tolerance in the installation opening dimension versus a 3 cm installation clearance is certainly not insignificant, especially since the tolerance in the opening size can be both positive and negative. With aerated concrete this is easy to rework, but with porous bricks or sand-lime bricks it is more delicate.