Any time you shorten the overall length you are reducing the volume in the case below the bullet. This increases the pressure when the round is fired. When you can't find the exact load in a book for the bullet you want to load you can check the difference measuring the bullet you want to load compared to the overall length of the one in the book. An example is you want to load a FMJ bullet but only have data on a JHP with the same jacket. When measuring the JHP you might find it is .010 shorter then the FMJ. You can start your load with the OAL .010 longer. The point is, if the bullet is the same weight and the base shape and jacket are the same, you only need to make sure you are seating the base to the same depth. This is still a round you really need to start at the minimum load and slowly work up to and amount of powder that works.
The available volume in a case for powder is very important. That is why you want to make sure the crimp is correct to prevent set back of the bullet. If you seat the base of a bullet .020" deeper in pistol ammo you should reduce the starting load about 0.2 grains. The amount you have to reduce the powder will vary with the type of powder and the size of the case/caliber. This is the reason people are telling you the shorter OAL you need to use and the amount of powder should be less.
When the OAL is longer you can use a bit more powder as well. I like to error on the safe side and will go to a slower powder and start over working up a load when I get very close to top end loads. I generally load faster powders with lighter bullets and slower powders with heavier bullets. I also tend to use faster powders to load softer shooting loads. I like shooting soft shooting ammo when I plan to shoot a lot of rounds at a time. It is also more pleasant for a new shooter to fire soft shooting loads. The best part of loading your own ammo is you can tailor it for you and your needs.