I've used a fair bit of Unique for light 38 Spl loads. Something in the 4.0-4.5 gr area will give you velocities ranging from about 750-800 fps. I used Blue Dot many years ago and recently started using it for Cowboy Action Shooting as it gives off a fair bit of smoke without the black powder clean up. I use 5.8 grs but safe loads range from 5.6-6.1 grs. One problem with loading to the lowest possible data is that one begins to get into erratic performance. It is quite possible to get a squib load on one round and a dangerously high spike on the next as one suffers detonation. This is believed to be when the powder drops below the flashhole allowing the flame to ignite across the length of the case. This leads to a too fast burn and pressure spikes. Best bet is to use closer to maximum charges with most powders and better yet would be to switch to a different powder such as "Trail Boss", "Titegroup", or some other bulky powder to lessen the odds of this happening.
I found out by accident my 4" M15 S&W shoots real well with the old 158 grain RNL 'halt or I'll dent your paint' load.
So I've been loading up 158 grain RNL bullets with 2.7 grains of Clays. According to the references, that should give around 700-750 fps. It shoots very well and has very mild recoil. Double check that load before using.
If you don't want to buy any new powder right now, check your load manual and try a few rounds of the starting to middle loads with whatever powder you have. Chances are it will do well by you.