So Targeted Bait is complicated. It actually has 5 effects, of varying levels of importance.
1) The Locational Chance is multiplied by 1.66.
2) The spawn multiplier is increased by 1.66.
3) That Spawn Multiplier is no longer capped at .9, and thus it's possible for common fish (like the Sandfish) to be a value of 1 and thus never fail the check.
4) Instead of getting the first fish on the hook which passes both of those checks, the game waits for 3 fish to pass both checks, and if the targeted fish is among those 3, it's on the hook. If it's not, then you get the 3rd.
5) The game normally, if no fish on the fishing queue get on the hook on one pass you get trash. With targeted bait active it goes through 2 passes if you haven't gotten 3 items which passed both checks in the first pass.
The Locational Chance is the number shown in Locations.xnb in the game files for the fish in question, in order for it to show a random number chosen between 0 and 1 must be less than that number. But, for most fish in most circumstances, it's already 1. So it'll never fail the check anyway. There are some exceptions, Lava Eels in the Caldera have a chance less than 1, and the farms generally do for the locations they evoke. The Beach Farm, for example, has a general .66 chance. So in addition to passing the spawn multiplier check, you need to pass that location check to get the fish on the hook. Without targeted bait, you fail that check 1/3 of the time. With it, you always pass, since .66*1.66 is above 1. But, beyond those specific situations, this has no impact.
The spawn multiplier is increased by fishing skill and is defined in fish.xnb, and it ranges. For an Octopus, it's .3 at fishing level 10, for a sandfish, it's .85 at fishing level 10. Times 1.66, so for an Octopus it's now .498 and for a Sandfish it's above 1, but would normally be capped at .9, were it not for targeted bait. Same deal, a random number less than 1 needs to be less than that to get it on the hook.
Sandfish in the desert - Normally, you catch it 53% of the time. With this, it's guaranteed to pass its check if it gets to it. So the only way it fails to get on the hook is if you have it 4th in the queue, and all 3 other items (river jelly, algae, and scorpion carp) pass their checks. That's 1.3% of the time. It's not guaranteed, but it's really, really close.
Octopus at your beach farm - Normally, you catch it in Summer in the morning around 6% of the time. With this, it's likely closer to 40% of the time, eyeballing it. But again super complicated and challenging to actually calculate.
Why not just set it to guaranteed? Well, that's not how he chose to do it. Don't know what else to tell you. It would be rather overpowered if it guaranteed it and less interesting too.
But if you want to know what fish have what odds, and what their spawn multipliers look like as well as how many fish are in the queue, well, I happen to have a sheet which outlines that data (and has the fish.xnb data in full in it) here:
docs.google.com
Hope that helps or at least proves interesting.
