Wow, that's a lot to answer. Firstly, a cronograph will not keep you safe. It will give you load velocity and sd. SD is the deviation in velocity between each round of the same load. In other words, if you shoot three rounds of 9mm 125 grn jhp with 8 grains of hs6 and they crono 1500, 1505 and 1510 sd is the difference in the three. The smaller the sd the more consistant the load and the more accurate it should be.
One way to avoid dangerous loads is to closely watch the condition of your fired brass. Look for excessive primer flattening and split or overly stretched brass.
Start with 10% below published loads and work up slowly. Use the crono to monitor velocity and sd. Check brass condition. Remember different powders react differently to heat or cold. Some spike pressure in the heat and some in the cold.
When in doubt, don't load it. It's not velocity that indicates internal pressure. Companies use strain guages to develop safe loads and data for publication.
. If you're not a very experienced reloaded, go slow pushing the envelope. If you don't have a good reason like shooting major loads for uspsa open, published loads are fine.
I hope this helps some and guys, correct anything I got wrong.