I go with 3.5 grains of TG for 124 gr. 9mm bullets (Berry's plated RN), which delivers very acceptable groups (approx. 2 inches @ 10 yds off-hand) out of my Shield and Glock 19. To put a bit of perspective on that, I'm not an ex-LEO or military service shooter; I'm just an average joe.
In 9mm it's pretty easy to see a double charge of TG, which has happened a couple times for me (7 grains will pretty much fill the case almost completely full), but was also easily spotted and fixed. In .45 auto, or .38/.357 etc., it is more difficult to spot a double charge. For kicks I once dropped 4 charges into a .44 mag case and still had some room at the top. Obviously, I didn't put a bullet on top of that.
As for metering, through my Lee PPM, I consistently get a variance of only +/- 0.1 grains (I've produced a little over 2,000 9mm rds with TG to date). If I'd tracked it, I probably could've counted the number of charges on both hands that were greater/less than 0.2 grains. So IMO, TG meters very consistently.
About the only thing that I'd consider a negative about using TG is that it does leak out of my PPM more than most other powders I've used, but in all other respects it seems to work just fine for 9 mm.