Micro man,
The general formula for seating depth is:
Seating Depth = case length + bullet length - COL (Cartridge Overall Length*)
What you are trying to do by controlling seating depth is control how much space the powder starts burning in. If case lengths vary, but head thickness is consistent, then using the formula with each case length is going to produce varying powder space. The best practice is just to use the SAAMI maximum case length and ignore your actual length as this will put the bullet base a consistent distance from consistent head thicknesses, making powder space consistent. With the .45 Auto, that SAAMI length is 0.898". So the formula becomes:
Seating Depth = 0.898" + bullet length - COL
Once you have found the seating depth for one bullet, you then find it for other bullets of the same weight and construction** but different length by rearranging the formula to:
COL = 0.898" + new bullet length - seating depth
One caveat with all this is that not all cases do have the same head thickness. Wall thickness can also differ below the normal range of seating depths, as, for example, the reinforced case made by Starline for 45 Auto +P loads. The arithmetic works precisely only with cases that match the cases used in the original load data. The way the difference is handled is to have you always start with the lowest load in the tables you are using and work up in steps that are about 2% of the maximum charge weight on the table, watching for pressure signs.
A second caveat is that some bullets, seated to the correct COL to match the seating depth of other same-weight, same-construction bullets, may not result in smooth feeding. If you have to seat a bullet deeper to get correct feeding, start the load workup looking for pressure signs all over again. In some instances, this will make an important difference. In some, it makes no difference because the primer is unseating the bullet before the powder starts to burn, making the actual powder space bigger than as-loaded. This is why you are stuck with working loads up to see if they are right in your gun.
* COL, COAL, OAL all mean the same thing. The letter "A" in some initials harks back to pre-1950s spelling when overall meant "taken altogether" while over-all meant a total length. The first form has the initial "o.", while the second has the initials "o.a." You see the hyphenated spelling in pre-1950 dictionaries. Still, by the time Webster published its 3rd edition in 1961, the hyphenated form had been dropped, and both meanings had come to be spelled as the single compound word overall. So, today, most load manuals use C.O.L. and the obsolete pre-Korean war spelling initials, O.A.L. and C.O.A.L. are used in fewer places. Often, manuals drop the formal inclusion of the periods between letters. But any of those forms work to communicate what you mean to other reloaders.
**Same construction means both cast or both jacketed with lead core or both copper solids or both moly-coated, etc. Different types of construction take different amounts of pressure to start them into the lands of the rifling, resulting in different peak pressures.