No problem. Headspace is a SAAMI spec that measures the distance from the shoulder to the flat of the rim. Each caliber is different of course and belted magnums headspace to the belted rim. There are go/no-go gauges that gunsmiths use to check headspace. Normally you would have this checked if you have problems like case separations or other unusual signs of pressure/case failures. And that assumes that charges and other variables are eliminated. COL is a different animal. The recommendations in the books are just that. The minimum is the first to pay attention to. Less than minimum could result in high pressure. Max would be jammed to the lands or magazine length limitations. Higher pressure could be observed at the min or jammed to lands. As you may notice in the books, the COL changes with bullet weight or style. Playing with different COLs can sometimes affect accuracy but there are other factors too. Just depends what your goals are.
As far as being able to tell how you are on the lands, that can be a bit complex. Some will create a dummy load with a case sized just enough to hold a bullet a long way out and then is chambered carefully to have it contact the lands and shove the bullet into the case. This is great as long as the bullet comes out and does not stick to the lands and back out of the case when the dummy cartridge is extracted. Another method is to use smoke or a whiteboard marker on the bullet of the dummy round and with trial and error on seating depth of the dummy round you can tell when the bullet touches the lands when the lands make marks in the black coating on the bullet. There are also some precise seating dies that can be used if you know the specific length of that distance in your chamber. I am certainly no expert on that. I suggest you do a specific search of this forum on that subject and you will be sure to find some interesting reading on that topic.
