On another tack there is a rule of thumb that the first 1/2 of the Rad does 3/4 of the cooling - is there some truth to that claim? 

George (down under)

It could be roughly true.  The heat transfer is proportional to the temp difference between the core matrix surfaces and the air.  As the air heats up going through, that difference decreases, and the heat transfer is reduced.  To make the most effective use of the core volume, you want the air to still be cooler than the matrix at the exit – by maybe about 30- 40F at the design point (another rule of thumb) but is dependant of other conditions.  So if your design point is high power climb with max coolant temp at say, 210F; and you want to do this on a 90F day, then I guess you could design for the max delta T through the rad of about 70-80F.  Since I dump the rad exit air into the cowl, I chose to design for a 50F delta T to give me some margin, and keep the in-cowl temps down.

Al G