I didn't measure compression because I figured it was pretty definitive between the coolant loss with no trace, high mileage, oil in reservoir, a cool system pressurizing quickly, etc.
When I fill the car with coolant I usually do this. Remove coolant temp sensor, fill the car with it off until it spews out of the flange on the head. Put in coolant temp, remove upper rad hose from rad, fill that til it has coolant, re connect. Then I top off res, and start car with heat max. Usually run it for a good while, massaging hoses. When its cold, replace what's missing.
However, these diesels do run cold, on average mine runs a hair or so above where the lowest white on gauge is. I have an aftermarket fan with a temp probe which kicks on very early. It could be a possibility, but I am wondering why it would be that low after all these times refilling it, I would adventually think it would be full.
Not to mention the concerns I listed above with the coolant pressure and such.