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.
Since Beer Smith added in the BW model it has been much better and pretty close to your spreadsheet. I use your sheet and Beersmith both to confirm. Thanks by the way your sheet is awesome.