Working pressure of a boiler is 470 psig.the safety valve lifts 570 psig and blows down to 480 psig.the blowdown is what percentage og the working pressure?

The way I read your question, it sounds like the blowdown pressure is 90 psig (570 psig - 480 psig). So that's 90/470 = 19.15%.

Am I missing something? It didn't say anything about absolute pressure versus gauge pressure, so it seems to really be that simple.