Ordered all my 9mm supplies.... Now Im overwhelmed.

I'm no 9mm expert, but I'll do what I can to help until some experts come along... We do need to hook you up with a local 9mm guy and get some good hands-on help. I am in HTB and am more of a .40 loader, but if I can be of help, let me know.

I like to at least look at the manufacturer's data. Unfortunately, Alliant's Unique data is pretty slim. They have a 124 Gold Dot Hollowpoint with a minimum OAL of 1.12 and a max charge of 5.8 grains and that is about as close as it gets to what you want to load. There is some useful info here though. Lead will typically make velocity (and pressure) with less powder (1/2 gr. is a good, but rough, rule of thumb for 9mm/.40/.45). Since 4.5 is well under 5.8 it should be fine. The minimum OAL is useful, too.

I looked at Hodgdon's data and it is a bit better: http://data.hodgdon.com/cartridge_load.asp

Hodgdon lists the cartridge overall lenth for a 124 LRN at 1.169 and for a 125 LCN at 1.125. This makes sense, as you assumed the flat nose will have a shorter OAL. I believe the LCN is essentially a flat point, see here which is my understanding as well.

Do you have any FMJ's or LRN factory rounds? Measure a few of their OAL's to get an idea, but 1.15 sounds in the right ballpark from the above for the LRN and should easily clear the lands. I'd be willing to bet you could go out close to the 1.169 and still clear. As to your question on what OAL to start the 125 FP for the same powder charge to achieve the same velocity, I don't think anyone can tell you that. Different bearing surface, different depth into the case, different bullet makeup, etc, equals too many variables. You can guess, but whenever you try a previously untried brand or style of bullet you will have to work up from the bottom again.

Hope that helps a little.