I'm starting paint work today and was wondering how perfect you make you weathering, are we following templates curve for curve, fleck for fleck, or are we roughly getting the shape and feel of the weather?
It's your lid, so ultimately its up to you. As Mullreel said, if your goal is perfection, then you are more apt to be happy with it once you realize you may not have achieved perfection, but a goal that is clearly in your comfort area. One other thing to consider, is that you can go back once the majority of paint is complete and add the very fine scratches and tiny flecks.
Also, make sure you have the surface area ready to receive paint. The chrome paint you had on there is going to be much more likely to have problems with paint adhesion if you haven't scuffed the surface with some fine sandpaper(400 or 600 grit wet sanded) or fine scotchbrite or 0000 steel wool.
Take the spatter on the ESB helmet as an example:
There are certain areas on the helmet where it's more apparent. I concentrate more on these areas, and then let some of it
carry over so it's sort of "feathered" out. If you have good clear reference pics, there are a few places where there are pretty
obvious spots of spatter. There are 3 streaks or drips on the left mandible, some good sized blotches on the outer corner of
the T-visor on the right side, and a few on the upper RF ear piece. Once I spatter my helmets, I go back with a small detail
brush (size 3/0 or 5/0) and dot in some of these spots by hand.
Here's the RF ear I mentioned above. On the left is a reference pic of the archive helmet with a couple of those spots identified.
On the right is one of my commissions from a few months ago.
The black spatter on the ROTJ armor is another example of this. Especially on the right chest plate.
Read up on superjedi's paint ups his skill and advice on his threads are priceless
It's up to you. As far as I know, you don't have to be perfect for the 501st, but match closely reference pics. What they say in their online guide is "The helmet is weathered and matches the visual references as closely as possible, both in colors and weathering patterns." Having not applied though, I can't be sure.
I go for "the illusion of perfection". I do my weathering by eye, which is probably not good enough for some people but it just has to be good enough for me. My thinking is that I want to make it look like it did in the movie (mine's an ESB Fett) but not be exactly like the movie. What I mean is, if it looks like it did, that's good enough for me, I don't care if it's the exact same part or material as long as it looks like it. That's why I'm ok with a lot of illustration board etc.
It also has to do with personal work style/method. I like to do it pretty good but done at first, then make improvements and re-do things over time. This allows me to have the fun of the completed costume earlier which helps me with motivation. I'd lose interest if it took me years or something to put together a "perfect" costume. Months is about all I can take without losing momentum.
Hope that helps.