Starting loads for unknown bullet
I started reloading last September, so I've had to deal with this question myself. Take the following for what it's worth, with a grain of salt, and as a place to start. Make your own determination as to how to proceed. Buyer beware.
One thing I did was to look online for loads (and OAL!) from people who had used the bullet, or something similar. I don't ever take that as gospel, but as a starting point, and then compare that to what the manuals say for a similar bullet to confirm it's not crazy.
Using that data, I'll start working loads up. I'll extend the OAL a bit from what was said for that load, and start low, though usually not at the very bottom. The low starting load, plus a bit more OAL, I feel gives me a margin of safety as I work up the load.
Even though you may think two bullets look the same, they may not have the same bearing surface (the part of the bullet which contacts the barrel). More bearing surface, more pressure. So starting low and long mitigates that possibility, IMO.
As I work up loads, I either increase the powder load, decrease OAL to the level suggested by my research online and in reloading manuals. Or both.
But I do one other thing: I run those worked up rounds through a chronograph. Paid about $100 for it. It lets me monitor velocity from my guns, and chart the changes in velocity for the different loads and OAL.
Ever since I started, I have thought it important for me to evaluate the actual performance of the rounds I produce. If you're working up a load, how do you decide where to stop? How much is too much?
I've compared my own handloads to commercial ammunition as a basis against which to compare. There's no way to evaluate my handloads objectively unless I have that chronograph data.
For instance, I have a good load for Hornady's XTP bullet in 230gr .45. It tops out at about 850fps, just where I want it to be. It's very accurate out of my XD. So for me, that's a production load. But I would not have confidence knowing what it is without that chronograph.
Buy a chronograph or borrow one, but IMO you'll find that you can make much better decisions about how you produce ammo if you use one.