You are absolutely right, data will vary by who produced the data and when. A lot of it is due to the differences in test bed, each publishers' test bed will be different. Some of it is due to atmospheric conditions at the time the data was produced ( a lot of this has been eliminated by climate controlled indoor ranges). My method for working up a safe load is to start by looking over all my manuals to see which one uses the components I want to use, if the bullet/powder combo is the same I start with the starting load and work my way up until I get the accuracy I'm looking for. I've never really concerned myself too much with the velocity of my chosen loads, I shoot them over a chronograph more to see how consistent they are than to confirm velocity.
While you're working up your loads pay close attention to pressure signs, inspect every piece of brass fired. I have no problem going over the listed max load to obtain the accuracy I'm looking for provided that I'm not showing any signs of excessive pressure (sticky bolt, cases getting stuck in revolver chambers, flattened primers, etc.). I usually try to start with the powder manufacturers data and that's my first stop in looking for data for a new load, however if they don't list my particular bullet I start digging into my manuals.
Reloading can definitely be overwhelming and a bit frustrating when you first start getting into it, but the pay off is well worth the time invested. I don't load 9mm but I have heard from several sources that it's not a particularly good cartridge to start out on, I imagine the pressure spikes talked about here are probably the reason. In all welcome to the wonderful world of reloading and feel free to post any questions you might have, seems to be plenty of experience here so someone should be able to help. I will also recommend thefiringline
forums, both have a ton of knowledge in the posts.