I find chocolate malt (at least Briess) to have a very mild toasty flavor as opposed to black malts that taste downright burnt if too much is used. If you have time and patience, I would propose some experimentation to find out what percentage black malt you can use before you can taste it -- this might be 1%, 3%, I'm not sure -- and then make up the difference in color using chocolate malt to darken it the rest of the way. This will probably take 2-3 batches to get it perfect, so you might want to do half-sized batches so you don't have too much to drink all at one time before you get it right.
If you are concerned about mash pH being too high or low, play around with adding some of the dark malts at the end of the steep or mash or whatever to limit this effect. Shoot for no lower than pH 5.2 in the steep/mash.