This is a complicated question, and without a thousand forecasters and millions of dollars of supercomputers to do predictions, you will always get fuzzy answers. Even with those, luck plays a part that can't be foreseen. Nevertheless...
There is a simple way to estimate your final place, a more complicated way taking into account your world ranking, and infinite refinements on that.
Caveat Emptor: This is an abstraction that produces an approximate answer. The variation around that approximation is very large. Someone familiar with international competitions and with your fencing will be better at predicting your chances than this coarse mathematical model. You may very well already know how you will do in Heidenheim as compared to Tallin, in which case this whole page will seem more amusing than useful.
The most variable quantity in this equation is how well you are going to fence. Anyone can lose to someone unranked in the world, and conversely even the most mediocre fencer can beat the top ranked fencer on the right day.
Since you have to put a number on this, we decide on the highest ranked fencer you can beat today: `T' (for Today).
If you have a T of 100, you can beat anyone ranked 100 and below, but anyone in the top 99 will beat you. If you have a T of 1, you're going to win the tournament, because no one can beat you. If you have a T of 400, you are going to be dead last because you can't beat anyone.
This is a horribly simplified abstraction, but it's the only way to get a handle on the situation and start predicting the future.
The simplest way to estimate your final place is to multiply by `P', the attendance: P*T is your final place at the tournament. Simple, easy, and wildly inaccurate. But it provides a first indication of what range you can expect to finish in. You might really end up two or more brackets higher or lower, depending on countless other factors.
Given your world ranking, what will your seed be at a tournament with a given SF?
If you are ranked X in the world, and the tournament has an attendance of P, you will be roughly seeded at place X*P.
(If you aren't in the FIE standings, you will be ranked randomly among the other unranked fencers, always somewhere near the bottom of the entries. It pays to have a ranking.)
What does this tell you? Most importantly, if you are seeded in the top 16, you get a bye right to the second day.
This is a huge benefit, since it guarantees you will fence someone seeded lower than you in the first and second rounds the second day, and you won't be as tired. You still have to win bouts to do yourself any good, since you can't get a world ranking that high with top 64 results alone, but it's a good start.
What's the strongest tournament I, with my world ranking of X, can get a bye in? For the person who is seeded 16th, X*P=16, so SF=64/X. If you have a world ranking of X, you will be able to get a bye at a tournament with a SF of 64/X or lower. Ranked 128th => SF 0.5 or lower. Ranked 64th => SF 1.0 or lower. Ranked 32nd => SF 2.0 or lower. Ranked 16th => guaranteed to get a bye.
There are a lot of details to cover when estimating how you will do the first day. You know your seed, and now you have to figure out what pool you're in, who else is in it, how you will fare against them, what seed that will get you out of the pools, and who you fence in DEs.
You can look at your world ranking, the estimated SF of a tournament, the estimated quality of your fencing that day, and see very roughly where you will place. This will provide you a guideline for what events are useful for you to attend if the goal is to acquire US or FIE points. If the goal is to gain experience you will use other criteria for picking events.
Read about what you get from different world cups.