I use a marker... never had any problems...and usally i do not write anything I just use a different color and put a dot big enough to see and write on the calendar what color went with what day... ( example... today would be Blue dot- 4 eggs- 3-30) something like that... I think its just as easy!
and if I run out of colors I will do double dots... and so on and so on ...
I use my son's old pre-k / kindergarten pencils, those big fat ones, they write so nicely. They have to be used sometime and he never used them and probably never will.