There's also the distinction between "I know that X% of this will (not) be useful" versus "I know which X% of this will (not) be useful." Especially when "usef

There's also the distinction between "I know that X% of this will (not) be useful" versus "I know which X% of this will (not) be useful." Especially when "useful" includes "to get a future job" or study something else much more useful and interesting later."