Adjusting Microwave Times
Without any other information besides "80% power", this really isn't a very well written recipe. If you don't know the wattage of the microwave used in the original recipe (i.e. how much is 100%?), you are in the situation of the musician in the movie "Spinal Tap", who prefers the amplifier whose volume knob goes up to "eleven" when all the other amps stop at ten. 

But it doesn't matter anyway if you have no way to adjust the power of your own microwave. You will just have to experiment with different times. This is the case whenever you use a microwave oven for anything. As the saying goes, "microwave ovens vary". The prescription given by Paparazzi, ...[ 3 minutes X 0.80 = 2.4 minutes ] seems like as good a starting guess as any. Test the lemon squares a little before 2.4 min., &amp; keep cooking in increments until they're good, then record the final cooking time for the next batch.