This is a matter of experience rather than exact measurement. There are areas that just shouldn't be cut, and others where considerable material can be removed. Most of the machinists that do this work aim for a particular weight reduction from the way to heavy 22 pounds. It is not hard to get to 15 pounds. Check with Richard Jemison, Alfar7 on the BB. He has one or two giant lathe's and likes to make these lighter. Unless you have a big lathe, and some junk flywheels to practice with, this job is best left to one really familiar with lightening them. Clutch and flywheel blow-ups are always VERY destructive.
I gave up lightening the iron and nodular iron flywheels in favor of aluminum flywheels some years ago. One properly lightened just slightly, failed due to an air inclusion right outside the bolt circle. The clutch/flywheel assembly danced around in the bellhousing like a spinning top, completely ruining the bellhousing transmission and case. Fortunately, it remained contained, the car and driver were spared.
On the other hand, I ran a 14 pound lightened iron flywheel, cut by Ron Neal at AUSCA, for many years behind my GTA engine. I never had any problems.
Have your 1600 wheel done by someone with experience, or go with aluminum.