Quantifying Simplified Language

If you wanted to be able to quantify how much easier it is for users when you reduce the reading level of you text, how might you do that?

For example, how would you figure out how much longer it would take someone who reads at a grade level of 8 to read a very similar text written at a grade level of 10, maybe 20% longer? I know that's assuming they understand the text written at a higher level (which is not a good assumption), but if they did understand it, we'd like to know how much time would it save them not to have to figure out the harder text.