There is no such thing as a velocity flat spot. When you add powder, it goes faster. To see it for yourself, load the same 1 shot per charge ladder and do it multiple times, it will be a steady line increasing in velocity as charge weight increases.
How in depth your load development is depends on what you are looking for in terms of accuracy and precision and how capable your system is of producing that. (Bullets, barrels, brass, rests, scopes, shooter skill etc..)
Where you start also depends on what information you have. There is so much info available out there for so many different bullet/powder combos, research it a little. Pick the bullet you want to shoot and see what are people using for powders/primers/brass. If you have quickload or Gordons reloading tool, you can also get an idea from that AS LONG AS YOU PUT IN THE CORRECT INFORMATION. A little rant, too many people use this incorrectly and then exclaim to the internet how someone else is loading so dangerously etc.. Well, if you put in the wrong information to the calculator, you'll get the wrong result. If you put my powder charge in a 308 case at 2.800" mag length, well, it wouldn't be able to fit first of all, but would also tell you it's probably 90,000psi. Well that's just wrong, since my OAL is almost .500" longer, the pressure is under 58,000. It's still a .308, just not your .308. I'll say it again IF YOU USE GRT, YOU MUST HAVE ACCURATE INPUTS.
On to the real question, I suppose. Once you pick you bullet, brass, primer and 2+ suitable powders, a single seating depth, figure out a safe starting point and load a one shot per charge "ladder" (For anything like a 308, no need to go less than .5gn increments). Shoot the ladder either at the same aim point (and keep track of bullet holes per charge) or separate aim points next to each other. As you do this, you will see the point of impact shift as the charge increases as well as keeping an eye on pressure signs. You should see a point where ~3charges form a nice group at the same point of impact. Load some 3 shot groups in that area to confirm, then choose the lowest charge that shoots the best. Now you can play with seating depth if you choose, again in 3 shot groups, in .005" increments (if your system is capable enough, do a finer test in .003" increments then load to the longest). From here, you can go as far as you wish, if you want to play with different primers, primer seating depths, neck tensions etc.. If you do different primers, expect to also play around with powder charge a little. Once you really start dialing a load in, then you can start doing 5-10+ shot groups to confirm it will hold up. I don't like doing more than 3 at the start because it's just a waste of expensive components. If you shoot a big group with 2 or 3 shots, it won't get smaller by shooting more.
Being able to load at the range in some capacity makes this process much easier and you also don't need to possible damage bullets if/when you need to pull them from loaded rounds. If you can't take a scale with you, I use small glass vials off amazon that I pre-charged at set amounts and dump powder into primed cases as needed, seat bullets with a little Lee press and get to it. Also, keep track of everything. I have a Garmin chrono and use it every time I shoot, no reason not to. Keep track of how your loads changes with weather.
Just my opinions and works for me.