I usually fill up to 3/4 of the fire ring and use the old lump until it gets to about 1 inch of the top air holes in the pit. I do quickly stir the old lump go get the small pieces through the bottom grate. I will get from 3 to 5 cooks until I add more fresh lump, I don't bother mixing used with new.

If I think I need some new lump I put it in. No mix just cook. If I am going to do a long cook I will then push the old lump to one side and put fresh at the bottom, then move the old from the other side on top of the new. Put some new in and slide some old over ontop of the second side.

Really it isn't something I worry about too much. I have not had any problems with temps low or high.

Possibly it is a case of beig fat, dump and happy.