Is there a general rule of thumb for the excess amount of resin to apply to CF part above what is needed for saturation? e.g. if i calculate i need 500g of resin to saturate my part,  how much should i apply to my part during the hand layup?

Not sure I understand the question... Wet-layup is simple, you aim for a 50:50 fibre:resin ratio so you need the same amount of resin as your weight of reinforcement. So if you weigh your reinforcement and it weighs 500g, you need to apply 500g to wet it out sufficiently. Alternatively you can use the quoted gsm value of each layer of reinforcement you are using. For example if your part is 0.5sqm, and you are using 2 layers of 200g carbon and 1 layer of 650g carbon, you would need 525g of resin to wet out the reinforcement.

When mixing, you would probably want to add a 10% wastage factor for resin left in the bottom of the cup and in your brush etc, so you would mix say 550g.

But your question seemed like you already knew that, so I'm not sure what you are asking? You dont need nor want to apply excess resin. For wet-lay, this wont help your surface finish, it only adds weight.

It also makes a difference how you use your resin. The first layer should be applied very resin rich, and every following layer with less resin. If you use vacuum compression, the last layer could be applied almost without resin.
But if you use vacuum compression, I would not bother with wet layup anyways and switch to infusion, with gives better results for the same amount of work.