I don't think there is a generalizable relationship between velocity and psi as they differ between powders and components.
However, for the same powder and components you can see trends. In that Clay Buster load data I posted you can see what an additional 1 grain of powder does to velocity and pressure. You can back off or add have a good idea of what velocity or pressure will be, though since I am usually reducing loads so I am not as concerned. Especially, when some of those Red Dot 3/4oz. loads are already in the 6000 psi range or less.
I think it is good to be cautious and understand what you are doing. And there is a lot of good light load data out there. Since I shoot a lot of RSTs and reload the empties, I would also recommend the Cheddite Reloading Manual(the type of hulls RST uses).
I would pick a good load and try it out. You can make a nice light 7/8oz load from Red Dot as well. I have a Mec 600 jr for every gauge I shoot 10-410. For the most part, I stick with a powder I like for each gauge for target shooting. My Mec 10ga. and 12ga. always have Red Dot in them.
Here a link to another thread when I was experimenting with 2 1/2" 12ga. 5/8 oz loads which I enjoyed. A few other members chimed in with their ultra light recipes. I am going to load some 5/8oz 12s using 14grains of Red Dot for a 2"12 sxs I recently got that weighs 5lbs 2ozs:
http://parkerguns.org/forums/showthread.php?t=25879