I don't think you should get to obsessed with this sammy. When it comes to worrying about fluid levels I always remember my Father-in-law who was an "office type person" - worked for an investment trust. Knew almost nothing "technical" about his car. When Mrs J and I got "serious" I was dragged back to meet the in laws. I was well into my studies to become a qualified motor mechanic and when he learned that, one of the first car related subjects he broached with me was about the radiator level on his Wolseley (BMC "B" series engine) He was puzzled about why it needed topped up so often. Turns out that every Saturday he would check his levels (as many folk did back in those days - 1960's) and it would take a pint or so every Saturday to bring the level back up. I got him to go out to the car with me and show me what he was doing. Turns out he was topping it up to the top of the radiator filler cap - No expansion tanks in those days. Of course the first time it got up to working temp it ejected the expanded volume of water past the pressure cap and for the rest of the week ran, as it should, with the level about a couple of inches below the top of the header tank. When he checked it cold again the next weekend, cold of course, he would see this drop in level and conclude it needed topped up! The insidious thing about what he was doing was that he was constantly diluting his antifreeze. he was very lucky he never ended up with a frozen engine in the winter. He was the same with his oil level. It had to be exactly on the full mark and he would add, literally, an egg cup full to bring it back to bang on the full mark.
If your coolant level is steady somewhere between the MAX and MIN marks when cold, and not constantly diminishing, you don't have a problem. Ideally, as has been pointed out above, it should be bang on the full mark (d'you know I've never actually read the owner's manual so it was news to me that the recommended level is to the MAX mark) I've done many water pumps, thermostats, hoses, and other coolant system related repairs on many different makes of car during my life and, basically, as long as there's coolant in the expansion tank when the engine is cold, you're not going to cause damage to the engine due to lack of coolant. Of course it's wise to have a reserve in case a slow leak develops hence the markings (MIN and MAX) on most expansion tanks. In practice the level would need to drop significantly below the MIN mark before you would be deeply in the Doo Doo! I'm very "happy" and sleep soundly as long as my level is between the marks and tending towards the MAX. The big thing is to always check your level when the engine is cold and that it doesn't drop below the MIN mark but even more important is that whatever the exact level is that it remains constant when checked. If it starts dropping then investigate immediately before it has a chance to suffer overheating and if you're normal level is on the MAX that just gives you a greater safety measure in hand?
By the way. it's not at all unusual for the level to take some time (maybe a week or a couple of hundred miles?) for the level to stabilize after being drained and refilled like this because there are often little bubbles of air trapped in the system which take a wee while to work their way out into the expansion tank.
