I wouldn't worry at all. First of all, when measuring at room temperature, 5.4 is just about where you want to be. Water spreadsheets are just a way to estimate what your pH will be, and the actual result will vary, often by a lot. If you look at the lot analysis for malt online, you can see that even the same base malts produce mashes of different acidity from lot to lot. If you are trying to hit a certain pH and it measures higher than you want, use the calculator to estimate how much lactic or phosphoric acid you need to drop by 0.1, add that, and measure again. Repeat if necessary.