I use a stick with holes in it and hold a nail in the centre without pushing it in and a pen/pencil in one of the many holes and use it like a trammel then free hand around the pencil mark.
No Holes so that leaves me to decide which way around to use the blank all are sealed around the circumfrence and date and weight written on that as well as the wood. Saves having to pull them out to see which wood they are as well as knowing those odd woods that I will forget in the future.
I've got a curious question here for the more turny fellas than me.
If you're only going to store the blanks for a period to dry out, Is there really a point to cutting a blank round before you actually get around to turning it? I know if you're cutting up a plank for blanks it's more efficient on timber if you cut circles as tight together as you can but if they're already squares wouldn't it be better to leave them like that?
If you are cutting from Logs or Slabs, trimming them down to circles reduces the bulk taking up shelf space and reduces time wasted waiting for moisture migration during drying out of basically waste wood.
May seem trivial but by the time you've had to build yet another shed for wood storage it becomes significant.
I have a large perspex guid which I use to lay out the most efficient cuts for a board, these I mill myself with a chainsaw mill, then i just fee hand cut on the bandsaw and seal with wax till required.
Below link shows the rig and some of the blanks from it.