I’ve done something like that but my grid is 10x40. You might be able to adapt some of these ideas for a 10x10 grid.
The three techniques I used for it are:
My grid is organized into 10 separate 10-location memory journeys, each indexed with a number shape image.
My number shape images are:
- balloon on a string
- (I don’t need an image here – it’s the last journey)
So the first memory journey is marked by a candle, the second one by a swan, the 3rd one by a butterfly, etc.
Here’s an example of how I would find the 91st digit of pi:
Calculate the row: I divide by 40 to find which row the digit is on. 91/40 = 2 with a remainder of 11. That means it’s somewhere a little past the 2nd row. I can find the 3rd row by seeing which of the 10 memory journeys has a butterfly in front of it. (“Butterfly” is my number shape image for 3.)
Calculate the location: I put four digits per location. The remainder from the last step was 11, so 11/4 = 2 locations with a remainder of 3. That means that the requested digit is the 3rd digit in the 3rd location of the 3rd memory palace, except that it needs the following adjustment.
Adjust by 1: I have to make an adjustment, because to get the numbers to line up in columns with a monospaced font, I count the decimal point as taking up one digit spot. That requires shifting my digit position to the right by one. So instead of the 3rd digit in the location, I’ll use the 4th one.
That tells me that the image is the last digit of the 3rd location on the 3rd row.
To find the 3rd location, I look for the 3rd number shape image. It’s a butterfly that is hovering around a house in Berkeley (my 3rd memory journey out of the 10). I move to the 3rd position and look for the last digit in that location.
nnnn nnnn 4825 nnnn nnnn nnnn nnnn nnnn nnnn nnnn
The last digit in that location is a 5, so the answer is 5.
The way I did it also makes things line up with WolframAlpha, so people can type “91st digit of pi” into WolframAlpha on their phones to see if my answers are correct.
There’s a practice page here: