I don’t know how to do that with other systems, but I do that by making memory journeys of 10 locations each. I put four digits in each location. I then mark each of the journeys with a number shape image.
So if you ask me, “What is the 159th digit of pi?”, I divide by 40 (the number of digits). 160/40 is 4, so I know that the end of the fourth palace (with a small modification, see below) would be the 160th digit and then I can walk back one digit to find the 159th.
I can find the fourth journey by looking for the number shape image in front of it (a flag).
Because I encoded the decimal place (a butterfly dropping a stone on an otter), the first memory journey is filled like this:
3.14 1592 6535 8979 3238 4626 4338 3279 5028 8419
The decimal point means I have to shift everything over to the left by one digit. So to find the 159th digit, I look in the 160th digit’s spot, where there is a 5.
You can practice here. 