both of those blocks on the side of the mast should be used in the downhaul. there generally isnt part of the downhaul dead ended to the mast unless you are trying to make a cascading system.
however, looking at the other side of your mast there isnt the same configuration so forget what I said.... your mast isnt playing by the rules

The system you diagramed should give you 6:1 if I am correct which, depending on your boat/mast/sailing conditions might be enough but I have 8:1 on the Blade and had 12:1 (but now that i think about it, maybe it was 14:1 when i doubled it with the cascade)on the 6.0. I think N20s run at least 12:1?