Quick notes regarding luck manipulation:
It's easier to predict where is the best place to get a certain soul if you use the Memory Viewer (32 bit, Automatic Update) and jump to 0x2000000 - WRAM.
You can see the random number. Although unfortunately knowing its value won't help you in any way, you can see WHEN and HOW OFTEN is changes. So, the optimal places are those where it changes every frame. These situations are relatively rare, because the RNG is only activated when it's necessary.
A few examples of what triggers the RNG:
When you perform an attack;
When you hit an enemy (or anything);
When you do a backdash (random small clouds of dust appear);
When you destroy an enemy (or a candleholder) and it generates random flames;
When you use any Bullet or Guardin Soul with a random [visual] effect, such as Man-Eater, Siren, Ghost or Great Armor.
When you use a Familiar.
Also, the Minotaurs activate the RNG while they are standing (before they see you). The Skeletons, Great Armors, and the Gorgons always keep the RNG active (while they're alive, of course)
