I collect 3x2 plate blocks of the George Rogers Clark issue of 1929 (Scott #651). There are 288 plate and position combinations, so keeping a checklist is essential:

I thought the work I did building the table might be useful to someone else, so I'm sharing it here.
Most easily, if you want to maintain the whole thing with paper and pen, you can just
download a PDF file at this link that you can print. It contains an empty copy of the above table:

But if you want to maintain it on your computer, I've also
set up a Google Sheet at this link.
If you've never worked with Google Sheets before, and you have a Google account, here's a quick walkthrough of what you need to do:
1. Open the shared sheet. It will be in "View Only" mode; you
won't be able to make changes to it:
2. Make a copy by choosing "Make a copy..." from the File menu:
3. When the "Copy document" dialog box comes up,
give it a custom name:
4. Start filling out your spreadsheet with the blocks you have. To do so, highlight the cell that matches the plate and position combination you have, then click the "bucket" icon in the toolbar, and give the cell a different background color (I use the darkest grey that's not black – second from the left – but you can use whatever works for you.)
5. Continue until your table is filled out with the blocks you own:

Now, if you want to check the status of your collection (from a stamp show bourse, say) you can do so from any web browser or from your smartphone.
Or, you might want to print a copy to take with you. To get the best effect, when the "Print settings" dialog comes up, choose to print the "Current sheet", with no gridlines, including a document title, fit to the page width, in portrait layout:

That will give you a printed sheet that looks like this:

I hope this helps another George Rogers Clark plate block collector!