You need to comapare the comparison rate to Marriott's lowest rate, so if Marriott has advance purchased rate which is the lowest rate available, then you have to use that rate as a comparison rate. This is what I was told by the LNF team after a harsh struggle to get my LNF approved recently.

Don't keep trying if you failed once (if you have time to file one later), my experience is that once they turned you down, if you file another LNF shortly after that they will try their very best to turn you down. Wait a week and try again and you will get a better result.

As stated in previous post, if you reserve a rate from Marriott without breakfast, even if the comparison rate has breakfast included, you won't get free breakfast if your LNF go through.

Good luck.