I use the rule of thumb 12 of her own eggs, or the equivalant area for other eggs.. ie 4 goose eggs 9 turkey eggs and maybe 18 to 20 banty eggs..
you will get some people to say that it is ok to use sharpie pens or magic marker..   I am dead set against it..    use a pencil or crayon and be sure..
I only mark with a pencil..  I put the date of hatch...  no month, no year..
just the date number .. you will know automatically that it is the next time that number comes up what month and year it is..
I also have made up a tray chart for each tray I have..  I write where each egg is in each tray..  then I do not have to sort through the eggs to find any certain one..  I look at the chart and it tells me it is in row A thru F   of tray A thru F and in slot 1 through 12..  I can tell you everything I need to know about any egg..  each tray holds 72 chicken size eggs...
this is "must-do" for me as I do a lot of rotational hatching..
........jiminwisc........