Not what I meant, its a circle track around the middle cell that loads all the chunks of the farm at once
That's what I was thinking, although does it have to be the middle cell of the second layer (so middle of the whole farm) or could it just be a circle around the bottom middle?
things are only technically loaded within 128 blocks spherical around the player, for example my iron farm stretches from y:64 to y:256
which means that if i placed a track around the bottom layer the top 64 blocks wouldn't load.
i'm not sure about the size of your iron farm but the general idea is to at all times keep every cell within 128 blocks of where you are.
in my case i don't need a track and could stand at the center doing nothing because i designed it so everything was within 128 blocks from the center, but ridding a minecart is more entertaining and helps with making it automatic...
so to answer your question: if the upper most cells are more than 128 blocks from the bottom middle that you're talking about, then you would need to go higher, otherwise you're ok.
on a side note: the regular far render distance loads out 128 blocks which can make it easier to see visually what's loaded from where you're standing, however if you use optifine the "maximum" render distance loads beyond 128 blocks, though no mobs of any kind will spawn farther than 128 blocks.