It's roughly 1/7 chance as long as you avoid lightsday (no light /NIN proc). Kurayami is very hard to stick /NIN but you can try to spam it.
Best bet is to make a BLM friend. BLM/BRD and DNC/NIN you'll have between 5-6 out of 7 yellow procs on any given day and you can kill most everything with the right atmas.
Also, you may look into doing the quests for certain seals now. The drop rate on seals from quests, I believe, was greatly increased in the last patch or two.