nOval,
The COL is bullet shape and weight dependent. A bullet with a blunt nose will be seated less deeply into the case than a longer pointier nose bullet will be when the COL's are the same. It is actually seating depth into the case that matters, as that affects the size of the space the powder has to burn in.
To find COL, you need to know bullet length. For case length, use the cartridge case maximum from
the SAAMI drawing (0.898" for .45 Auto), and not your actual length, as it is the distance from the bullet base to the bottom of the case that actually matters in defining the powder space.
Seating Depth = Bullet Length + Case Length - COL (use 0.898" for 45 Auto case length).
Once you have that seating depth number for an existing recommended load, you can determine the new COL that gives that same seating depth with your different bullet shape by rearranging the above formula and using the seating depth you just found.
COL = Bullet Length + Case Length - Seating Depth
The two times this fails are: 1) When the bullet bearing surface (cylindrical full diameter portion) disappears below the case mouth. This means your bullet nose is too long to use for that COL, and you need it to be longer. And 2), when the bullet sticks out too far from the case and won't seat in the chamber because it's nose is too short for the calculated COL. It then has to be seated shorter. In that case you want to reduce powder charge to compensate for the tighter space until a load workup tests it to be safe at a higher level. For the 45 Auto, figure approximately 20% reduction of the charge for every tenth of an inch deeper seating, or 2% for every 0.010" inch deeper seating. This assumes same bullet weight and type (jacketed, cast, swaged).
That's ideal, though. In the real world I've seen pressures in short pistol cases fail to rise as much as seating depth would indicate because the primers seem to help unseat the bullet. So you need to reduce the charge, then test and work back up.