This is my personal method.......everybody has their own. Some ways are good others are not.
I start by making a thin line down each side of the spring with a Sharpie.
Start and stop the line about 3/8 from each end.
Use these marks to grind the spring evenly along the whole length about a 1/16 per side......do each side evenly and test till you get what you want.
Then file, stone, or polish out all the grinding marks and round the edges
Your finished
I personaly don't like Wasp wasted springs I think it changes the characturistics of the spring whereas narrowing the spring evenly maintains the characturistics and lightens the spring.
Thinning the spring is not a good idea ...again changing the characturistics of the spring.
Raven