I need to make a variable incrases in a loop system of blocks

Context:

I want to create an injector that when I reload it with 1 fuel item, injects a block 5 blocks below the injector, and if I reload it with other fuel item, block is placed 5 blocks below the previous block. And thus be able to create a loop system.

 (If you don't see the image: https://ibb.co/8cP0X9y)

So, I want to create a NBT variable that inserts the block every time an item is placed in the injector, but every time an item is placed, the NBT variable subtracts -5 (so the block must be -5 blocks lower than the previous one)

I hope this is not impossible haha