Psyching up for Summer, or, Why I Am Not a Math Teacher

This is my seventh year teaching summer school, so you'd think I'd have things mostly figured out. Why, then, am I struggling with what would seem to be a simple math problem? For instance, I was reading in the Course Outline of Record for English 50 that, for outside assignments (and I quote): "Students are expected to spend a minimum of three hours per unit per week in class and on outside assignments, prorated for short-term classes."
This is sort of confusing to me. Assuming I'm understanding this correctly, if students are expected to spend a minimum of 3 hours/unit/week "in class and on outside assignments," that amounts to  a total of 12 hours per week for a 4-unit class. Since students spend 10 hours per week in class during the summer, that leaves only 2 hours per week extra that they would presumably spend on outside assignments.

Not possible. Two hours per week would barely cover reading my syllabus. 

So I have to look at the phrase "prorated for short-term classes," and adjust my numbers. I emailed my department chair and asked for clarification on the amount of time summer school students might be expected to spend on homework for this class. Here's what she said: 

During fall and spring semesters, students would spend 4 hours per week in class, so they can reasonably be expected to spend a minimum of 8 additional hours per week on homework outside of class.  For summer classes, this does require some translation about the "prorating," since summer squeezes a sixteen-week semester into a six-week session; the basic formula is that you can expect them to spend at least twice as much time on homework as they spend in class.

This made sense to me upon first reading. If I can expect my students to spend "at least twice as much time on homework as they spend in class," and if they're spending 10 hours a week in class, I could reasonably expect them to spend 10 hours ("twice as much time") outside of class doing homework for my class. 

But then I read it again. And I got confused again. Does "twice as much time on homework as they spend in class" mean 20 hours (10 + 10) total, or does it mean 10 hours (in class) plus twice as much (20 hours), which comes to 30 hours per week on English 50 related work? 

That seems a bit much.

Now I'm really confused. Is this a math problem or a semantics problem? 

I emailed professor friend with these same questions. I tried to justify my fixation on this by explaining that I want to clearly convey my expectations on the first day of class so they know exactly what they're getting into (I didn't mention that what I'm really thinking about is how often students complain about the amount of homework I assign on Rate My Professor).

His reply: "I'd forget the equations. You expect them to do as much work as during a regular semester. I tell  my students that the first day. Like you say, so many think six weeks means not much work. Wrong!" 

He's right, of course! And by extension, I'm right, too! Students who take summer school should expect to do more work outside of class, not less! The should not be surprised with the amount of work they're assigned!

I've been tweaking my syllabus all morning, but after all my calculating and all my pestering, I think I've got the wording I want:

Because of the accelerated nature of summer school, students are expected to spend twice as much time on homework as they spend in class. Plan on allocating approximately 10-15 hours per week on English 50 homework this summer at minimum.

I italicized "at minimum" so students who are paying attention will understand that it's possible--probable, even--that they might spend more than 10-15 hours per week working on my assignments. 

So that should do it. Summer syllabus is finished. Check. 

(But I emailed my department chair again, just to be sure..).