In short, you have an unlimited scope to make it (I guess like all aspects of a custom made guitar) just the way you want it to be. As the heel is the start of the construction aspect that makes for the Les Paul's vaunted poor upper fret access, you have the opportunity to make this a bit less intrusive.
An almost flat version like the LP ultra pic above is also what a lot of the thin necked 70's guitars are like.
Have you worked out any plans? Downloaded any free ones? Checked out what is available for purchase? Frankly this is probably one of the dimensions you do not need. What you do need is to work out all the details you want before cutting wood.
With all the associated dimensions, which you can obtain if you search for 59 Les Paul plans for sale, although this will cost you some $$$. I worked on a set for a friend, so if you are interested in going that route PM me.
So for the dimension you are asking for above, it is not really that dimension you want but the dimension and details around the neck pocket (mortise) and then the profile of the neck at the body join (tenon).
As ARandal notes you can design this guitar any way you want, so modify as you require. I would suggest starting a build thread and let us know your thoughts, experience, and what you have available for tools and we can help out. Good luck.