There is no set measure for cutting a spring.
There is only one guideline... cut as little as needed. You can always remove more, but you can never put it back.
Knowing that a spring will 'settle'... its a good idea to cut about three quarters to an inch longer from your desired ride height. That way you can compensate for the 'settling' and not go too low.
I normally dont cut more than one quarter of an coil at a time when trimming for ride height. On my 6041's that were in my 67, I did an initial cut of two and a quarter... drove it around for 30 days to let them settle and then made my final trim cuts. The first quarter dropped about a half inch, the second about a half inch and the third dropped about an inch and a quarter. So... it goes to show that there is no set drop when cutting specific amounts.
But from a known freelength or a known 'new' spring... you can sometimes make a gross cut to get to your desired freelength a little quicker. For example, a MOOG 6041 right out of the box can take an initial cut of two and a quarter coils off to get it in the ball park for further ride height trimming.
The only other thing that needs some peticular attention, is how you cut the spring itself. Basically, hot and fast... and not to allow the material to heat soak. A perfered method is to get a five gallon bucket, put the spring into it and fill it with sand. The sand will soak up excess heat and it also holds the spring steady for you while you cut. And... MOST IMPORTANTLY... let it air cool.
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