I've been setting my lye concentration at 30% for no particular good reason. The difference between that and 28% is insignificant. Water is the least critical ingredient as far as amount in recipe.
I've left the superfat at 5% because I'm adding FOs which contribute additional fat not taken into account in SoapCalc. Unless my math is wrong, adding 1 oz ppo of FO adds an additional 1/17 part (1 ounce FO in 16 ounces base oil plus 1 ounce FO) or an additional 6% of superfatting, bringing the total superfatting up to about 11%.
That assumes the FO base oil is saponifiable and also assumes CP soap where lye is in the mix.