I use cover your plate as just a part of deciding what I will give, howover, I have a cap on the reasonableness of what you are paying per plate as 7 course meals are an extra that you don't need to provide and mostly gets thrown away. As I mentioned, I give also based on relationship but I do consider relatives at the same level (i.e. cousin, 2nd cousin, niece, nephew, etc) when giving as well. So I wouldn't give a cousin I liked more more money but I would maybe give a best friend more money.
When gifting it is a combination of criteria. What I can afford is the least considered as I generally could afford more than I would gift. So I consider - the cost of wedding, the relationship with the couple, the level of inflation since a similar family/friend event, what I gave at other weddings, what people gave me at my wedding, what people have gifted for my DD, etc... It isn't an exact science, I try to both be fair amongst friends and not punish those who are more frugal as I don't care about the fancy carp at weddings and would be happy to attend a reception at McDonalds as much as I would a fancy wedding. So for that reason, I have an average cover your head for the area ($100) and if you choose to serve lobster, I won't give more just because of that. I also won't give more just because I am in a better financial situation that others. You would think no one would expect that, but in my experience they seem to.