(No sparge) Have always used acidulated malt but wanna switch to phosphoric acid 10%, beersmith tells me I’d need between 10-15 ml of it to get down to 5.4, is this too much acid? How much acid will it take before you start tasting it in very pale beers?
Don't worry about the seemingly large volume of acid being recommended. That 10% phosphoric is mostly water and that's the reason why you need to add what seems to be 'a lot'. I'm not sure if the acid calculations in Beersmith have been refined, but I do recommend that you double-check the acid additions with another brewing calculator to confirm that 10% phosphoric addition. In any case, its unlikely that you'd 'taste' that acid since its one of the most neutral tasting options.