I use it in 9 mm 124 grain FMJ. 4.4 grains produced a rather light load that was accurate. I use a well lubricated pistol for these rounds and the cases were hitting the ground a couple feet away.
4.8 grains would cycle any pistol I have even with a new slide spring. There was a noticeable increase in recoil over the 4.4 load. I finally went with 4.6 grains and used an OAL of 1.125. This produced a load I am very happy with.
When I made some using 4.8 grains of powder I was also using 1.130 for OAL. Using an even shorter OAL might start causing pressure problems. The limits you can use are close to those for W231 powder with one exception. You can use a little more W231 powder then you can WST for the max load. As an example the max load for W231 was 5.0 grains of powder for a given bullet and OAL. I stopped loading at 4.8 grains of WST for the same load. WST does fill the 9 mm case quite nicely. I doubt you will ever get a double charge to fit in the case.