So this master card idea is really just a way of taking the information encoded by the master card, which is 6 bits (2^6 = 64, enough to specific a specific card in a 52-card deck) and scattering it across images that come after that so that you don’t have to have an image for the master card itself. You can scatter those bits any way you want, as long as there are six of them.
If you had even more images, you could do better, of course. At 8 Ben systems worth (8 * 52 * 52 = 21632 images), it is possible to do even better, because you’d be able to process 5 cards at a time (1 master, plus 2 pairs of regular cards), so efficiency of 5 cards in 2 images. An example of master card use for that would be:
Look at suit of master card and whether rank is even or odd, and that tells you how to encode cards 2 and 3:
spade even = BenA
spade odd = BenB
heart even = BenC
heart odd = BenD
club even = BenE
club odd = BenF
diamond even = BenG
diamond odd = BenH
divide the rank of the master card by two and discard any remainder. The result will tell you which system to use for cards 4 and 5:
0 = BenA
1 = BenB
2 = BenC
3 = BenD
4 = BenE
5 = BenF
6 = BenG
(You never encode cards 4 and 5 with BenH because there isn’t a rank above 13 (King)).
Basically same idea as I mentioned before. Upon decoding, the image for cards 2 and 3 will tell you the suit and the parity of the master card, and the image for cards 4 and 5 will give you the exact rank…
A similar idea can be applied for decimal digits. Let’s say you normally use a 3-digit system but you can’t imagine getting to 10,000 images, so a full 4-digit system is out of reach. You can only get to 4,000. Not bad, though. Here’s one way to take advantage of it, by observing that having 4,000 images is like having 2 extra bits per image to work with over having just 1,000, and that a decimal digit can be completely specified by 4 bits (because 2^4 = 16 is bigger than 10.) You scatter those four bits of a “master digit” among the next 6 digits.
method:
Convert the master digit to binary:
0 = 0000
1 = 0001
2 = 0010
3 = 0011
4 = 0100
5 = 0101
6 = 0110
7 = 0111
8 = 1000
9 = 1001
Okay, time to encode digits 2, 3, 4. Which system to use? Look at the 1st 2 bits above:
00 = BenA
01 = BenB
10 = BenC
(BenD will not be used for digits 2-4. It’s not needed for decimal digits.)
Next, look the last 2 digits of the binary representation of the master digit. This says how to encode digits 5, 6, 7 in your sequence:
00 = BenA
01 = BenB
10 = BenC
11 = BenD
Now you can forget the master digit. You will process digits 7 at a time, but you get 7 digits in 2 images instead of just 6.
BTW, even at 2,000 images you could get something, but it doesn’t save much. You’d have to distribute the 4 bits of the master digit across the next 3 * 4 = 12 digits, so you’d only be able to forget 1 digit in 13, which likely isn’t worth it.
For a 2-digit system, let say you had 400 images before you gave up on repeating Ben. I’ll call them SmallA, SmallB, SmallC, and SmallD, 100 images each.
Master digit binary conversion as above, generally same as above except you process 5 digits at a time and discard the master, so that’s 5 digits in 2 images rather than the 4:2 ratio one would get would only 100 images. Since in competition the rows are a multiple of 5, this might be worth considering. But if you have 400 images, it might be really tempting to put in the extra work to make a 1000 image system.