One way to think about lye concentration percentage is it's the number of grams of dry NaOH in 100 grams of ready-to-use lye solution.
A 30% lye concentration => 30 grams of NaOH in 100 grams of solution. You'd make this lye solution by mixing the 30 grams of NaOH with enough water (70 grams) to make a total of 100 grams of lye solution.
If you wanted to try a 33% lye concentration, that's the same as 33 grams of NaOH in 100 grams of lye solution. You'd mix 33 g NaOH + 67 g water = 100 grams total.
As the lye concentration percent goes UP, there is more NaOH in the lye solution. It's stronger. I think that's the direction you are wanting to go. So as the others have said, go from 30% UP to 33%. Not down to 28% -- that would be a weaker lye solution.
Not sure if this will be helpful to you, but here's a table I made that shows lye concentration and water:lye ratio and gives suggestions for what to use when.
Lye conc vs water:lye ratio | Soapy Stuff
I'm no expert at what prevents ash, but what other people say is going UP in lye concentration tends to reduce the amount of ash.