"Although a heavy pack slows me down it is not as much as I would have thought."

think again though, about the cumulative affect of that heavy pack - you only carried your pack once on your test- now double that for each portage, and then throw in a big fudge factor for the 10th portage of the day, maybe for the third day in a row.  Though I agree with taking one heavy load for the first of two trips, and the lighter load for the second trip.

Double tripping the portages, I guess I use a 1 mph rate for the one way distance, more or less.  I'm a fast walker and can do the return trip for load #2 at better than  3 mph - a fast pace for a woods trail, and probably for the most part, can keep to a 3mph pace while carrying the gear (note that those are light loads though)

But I don't just use the 1 mph rate -- what I do is assume 10 min unloading, and 10 min re-loading - that includes moving the canoe to shore and tieing, and setting it back in the water to laod again.  No it doesn't always take that long, but over the long haul, it averages out - that gives me some time to take pics, or do whatever.  So I plan a minimum of 20 minutes, even for a simple 10 rod portage (regardless of the fact that it won't take that long) - then I add the 1 mph rate for the distance, more or less.  That way, if I take time to saw some deadfall out of the trail, or can't find the start of the portage, or the water is too low, or there are new beaver ponds, or the portage landing is really crappy and takes twice as long as normal to deal with, I have some extra time built in.  For me, over the 8 or 10 portages I'm likely to be doing in a day, it does average out pretty closely to what I've planned.  

About the same as using 3mph for paddling - it averages out over a long day.  And I rarely finish my day early, normally I'll be about on schedule or I'll br running behind due to headwinds, or difficult portage conditions.

I do what you are thinking of, that is, I plan an itinerary for each day so I know how to get from point A to point B in 3 days travel.  Then I fine tune to adjust - maybe for getting to a certain spot on day 3, I'll have to make day 2 longer than originally planned - maybe I'll have to get up an hour earlier, and or travel a lot later, but at least I have a plan that will get me where I want to be, more or less when I want to get there.  That works about the same for a solo trip or tendem or group trip, though group trips will definitely be slower.


edit: if that didn't make sense - for a one mile portage, I'd use 1 hour, plus the 10+ 10 minutes load unload time for a total of 1 1/2 hours.