I've been using a COL of 1.135 with P.D. 124 gr round nose without any issues.
After blowing all day fretting over this, I'm thinking that sounds like a good starting point. That gets the bullet ogive outside the lip of the case, and doesn't make for feeding/chambering problems. I just wanted some logical justification for deviating from the reference round I was using.
Looking at all the loads I've done previously, and at a lot more loads in load manuals, I discovered something interesting. The seating depth on most FMJ pistol rounds is around one half of the bullet diameter. A few, like this example in the Hodgdon manual, seat significantly deeper, but very few seat less than that.