Yes, the requirement for prior approval is based on percent change in level of effort, not the drop in percentage points of level of effort. In this case, a decrease from 50 to 30 percent effort amounts to a 40 percent reduction [(0.5-0.3) / 0.5 = 0.4].
This is conceptualized more easily if you consider the effort level in person months. A change from 6.0 person months (50 percent effort on a 12-month calendar year) to 3.6 person months (30 percent effort on a 12-month calendar year) is a decrease of 40 percent [(6.0-3.6) / 6.0 = 0.4].