The short answer to this is, something is either an overlay, or it is not. It\'s not tough to figure out what the right price is for an exacta based on your own handicapping, and compare it to what the exacta is paying. The same thing applies to boxes, etc.
There is no reason to limit your overlay to the win pool only, especially with 20-1 shots, and an exacta involving a horse that is not an overlay (and sometimes even one which is an underlay) can still SOMETIMES be a significant overlay. What I try to do is get 10-1 on a 5-1 event, rather than 20-1 on a 10-1 event-- it helps with the spiking over the course of a year, and allows me to bet more on good betting races.
I do agree with what you have said in earlier posts about betting short priced horses. There is a degree of randomness in all this, and it since it works equally for and against all, it cuts against the short ones. There is almost always less edge there than people think.