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I think the calculation you used doesn't take into account the fact that you have some water flowing into the tank even when the tank is feeding the pump. So you are feeding water at a consistent rate of say 1.2-.45 in ball park terms. Your tank could be at least 30% smaller, probably more. You don't end up feeding the shower at 6bar.
It helps if you have a combination of cold and hot pressure from a hot water tank though. So that's 2 smaller tanks, one hot and one cold.
Always been this low. Problem with a 125 litre tank is that I literally have no where to put it.