Measure of short term memory

I’m doing a study on the impact of video quality and memory. The subject groups are going to watch a video of someone reciting 20 or so facts, and then will be asked to recall the facts or answer questions about them (this is what I need help with). The goal is to determine whether slowed down videos decrease overall memory or retention

I want a difficult enough memory task that most people will fall somewhere in the middle (I don’t want everyone to get 100%s as that’d mess with the math)

Anyone know of any memory measures that use a similar list of facts structure?