Going down by 3cm is not a wise choice. You'll be too low on your bars, your diaphragm will be constricted - lowering your lung capacity - and your hamstrings too loaded, just the opposite of what you want. Go down by 1cm perhaps but stop there............ Play with your stem if that is a concern.

Today's fitting/sizing "formula" is pure new age nonsense and not as efficient or powerful from established standard.