Good idea.

I'd do something like this:

x = (1*(x-1)) + (2*(x-2))+1 + (3*(x-3))+2

where x is the decoder lvl you aim for.

This would mean - to make a lvl 10 decoder you'd need:

1 lvl10 = 1 lvl9 + 3 lvl8 + 5 lvl7

the others would be made of:

1 lvl9 = 1 lvl8 + 3 lvl7 + 5 lvl6

1 lvl8 = 1 lvl7 + 3 lvl6 + 5 lvl5

1 lvl7 = 1 lvl6 + 3 lvl5 + 5 lvl4

1 lvl6 = 1 lvl5 + 3 lvl4 + 5 lvl3

1 lvl5 = 1 lvl4 + 3 lvl3 + 5 lvl2

1 lvl4 = 1 lvl3 + 3 lvl2 + 5 lvl1

now to start from scratch and only start with decoders lvl1-3 you would need:

1 lvl5 = 4*lvl3 + 7*lvl2 + 5 lvl1

1 lvl6 = 12*lvl3 + 16*lvl2 + 20*lvl1

1 lvl7 = 29*lvl3 + 55*lvl2 + 60*lvl1

1 lvl8 = 85*lvl3 + 133*lvl2 + 145*lvl1

1 lvl9 = 232*lvl3 + 378*lvl2 + 425*lvl1

1 lvl10 = 632*lvl3 + 1052*lvl2 + 1160*lvl1