I believe there is one question being asked and two answers being talked about.

The question as I see it is how far from the ankle should the edge of the ball be.

Fortunately we can et the answer from one of the all time best stylists ever to throw a beautiful gorgeous ball until the the day he retired.  The great lefty Dave Davis I was lucky enough to practice with when I first was learning to bowl lefty about 14 years ago.  One day he threw about 11 in a row in practice and one of his admirers started commenting on his striking.

Dave replied like this, "I didn't notice, but Thank You.  You see since 1957 when I warm up for 45 minutes, I always spend the first 10 to 15 minutes working on just my technique.  Would you tell me on the next bunch of shots if the edge of my ball is within 2 Inches of my ankle?"

From there he went on to tell what an aid this regular practice routine was to his ability to prevail in his many television matches with quite high pressure.  He went on to explain to me the solid leverage and adjustability that came form this form element!

He then went on to practice throwing shot after shot just inside of 2 inches from his ankle.

I got to experience exactly what he discussed in some of my occasional "on" form moments.

The other item of discussion is how many boards the center of the ball lays down from the slide board of the sliding foot.  This is the 7 or 8 boards discussed above by JustRico and he gives the Math of how to get there.

It is as  simple as the following.  Often if the slide board is 20 an example of this would end up with the lay down board of the center of the ball passing over the foul line being 12.

As to being closer that 2 inches to the ankle there are a few elite strokers who do that but it is really not necessary or dramatically advantages to do so and can lead to injuries or bounce offs as famously happened to a very good bowler named Storm DeVincent years ago in a PBA stop.

Right near or slightly inside 2 inches will accomplish all the benefits of foul line stability, balance, leverage over the release and the resulting adjustability at the foul line because of this solid foundation!. Practice it, own it, enjoy the benefits. 

Regards,

Luckylefty
PS It might be time for me to try to own it again also!  Mastering this fundamental for even brief periods of time can be for a lot of fun and profit.