For a lot of handgun rounds the advice to "work up a load" doesn't really mean anything and will only give you a false sense of security. You could work up a load into .357 pressure ranges and you still wouldn't have any pressure signs. The gun will be taking a beating it wasn't designed for but any signs of excess battering won't show up until it's too late.
Realistically all you have to go by is the reloading data. At some point you have to use some common sense. Do you really want to run a load through your gun that is pushing a 125 grain bullet 1,230 FPS? FWIW my Lee load manual also gives 6.3 grains of N320 as the max load with a 125 grain plated bullet. So with 2 published sources does that confirm the load is safe? Or does that just mean Lee copied the VV data? Most factory 125 grain regular (non +P) ammo is running about 900 FPS. Even the majority of +P ammo is claiming slower velocity than that with most loads claiming around 1,000 to 1,100 FPS. Only the hottest premium ammo is going near 1,230.