I think you are on the right track and should continue your quest. As mentioned, older manuals listed the seating depth instead of an oal... for a reason. I think the oal developed when there were only a few types of components available and people were trying to match standard factory loadings to save money. Since then, the garden has bloomed.

For many bullets I do exactly what you are talking about. If there is a crimp ring then I try to use that as my guide but still be aware of how much lead is in the case when looking at load data from different sources. For most semi auto bullets, I try to calculate my OAL based on a reasonable estimate (aka guess) of how much shank the selected bullet has, how much of that shank is in the case and what the 'normal' OAL of that caliber is. My OAL will fall where it may but is usually in the acceptable normal range. You will be looking at balancing pressure, mechanical functioning and velocity for charge/bullet weight when doing all this, so there are affects in multiple directions by changing it (that's why you need to understand the ramifications). Measure a few different bullet shanks to see what I mean and pull some factory ammo to see how deep they are seated (after measuring OAL). Take a look at this thread, where I did just that.

A couple of obscure 9mm loads

For 9mm, depending on the bullet, I find a seating depth between .18" (XTP) to .28" (some 147gr JHP) into the case gives me acceptable performance and OAL in the above areas. Some variance will apply to special cases such as short bullets, hollow base or longer projectiles with a long shank.. YMMMV