Most goal times come from somewhere vague: a round number, a friend's time, last year's result plus a bit of hope. Your own race history is a much better place to start, as long as you read it honestly.
Start with what you've done
Line up your last few races at the distance, best to slowest, with the splits if you have them. Then add a note next to each: the course, the weather, and anything that went wrong.
A hilly course in the heat and a flat one on a cool morning aren't the same result, even at the same fitness. A flat tyre that cost eight minutes isn't a sign you've got slower.
Look at the splits as well as the finish. If your swim and run are steady from race to race and the bike swings by ten minutes, the course or the wind is probably the story, not your fitness.
Clean the data before you trust it
Take out races that don't belong: a DNF, a day you were ill, a race where something broke. Adjust for the obvious ones you can measure, like a swim that was cut short or a run course that was long. What's left is a fair picture of what you can do.
In LankFit you can mark a race as did not finish, and log what happened on the day: a puncture, cramp, a wrong turn, with the time it cost. Those races then stop dragging your projections slower.
Using races at other distances
If you've never raced the distance, a race at another one can stand in. Runners have long used a simple rule of thumb: each time the distance doubles, the pace gets a little slower, not the same. It's rough, and it gets rougher the further apart the distances are, but it's a reasonable first guess.
It works best between distances that are close, and for athletes who've trained for both. A fast 10K says less about a marathon if your longest run is 15 km.
For a triathlon, build it leg by leg: a swim time, a bike time and a run time from recent efforts, plus your own transitions. That's how LankFit projects a finish, from your best recent efforts in each leg, made slower the older they are.
Course and weather belong in the sum too. A goal for a hot, hilly race should be slower than the same fitness on a flat, cool one. LankFit allows for the course and the forecast where it knows them.
Adding the ambition
Once you have an honest starting point, decide how much faster to aim. The time to the race matters. A few months of good training can win back a few percent. A few weeks can't do much at all.
A good goal sits just past what your history says is likely. Far enough that you have to train for it. Close enough that a good day gets you there.
Be wary of goals that need everything to go right. If your plan only works with a perfect swim, no wind and a run you've never run before, it's a wish, not a goal.
LankFit's goal outlook does this sum for you: where you'd finish today, the gap to your goal, and what the time left could plausibly win back, read out as anything from looking inside your goal to out of reach for now. It's a guide. Sleep, illness, weather, course, fuelling and pacing all play a part on the day.
Check it again halfway
A goal set in January isn't sacred in April. If the projection has moved, move the goal, in either direction. An honest goal you adjust is more useful than a brave one you quietly stop believing in.