The safe range of room-temperature mash pH is about 5.3 to 5.5. That indicates that the pH in the mash is 0.2 to 0.3 units lower, but that doesn’t really matter. The only thing we need to focus on is the ‘room-temperature measurements’.
A 5.4 room-temperature pH is a good all-around target. If you want your beer a little sharper or tarter, aim a little lower (5.2 to 5.3). If you want your dark beers to be a little softer, aim for 5.5. But don’t get carried away with a high pH target. All kinds of things go wrong when you exceed about 5.7 to 5.8. You can be a little low with pH and the beer should taste OK. But it won’t be so good if its a little high.