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