Each car seems to tune a bit differently so there's no cut and dried tune parameters to suggest. A custom tune, on a dyno by a DSP authorized CMR tuner, is done in phases, balancing quite a few engine parameters including STKR and a whole host of others. The run the car, tweak the program, run it again, tweak it again and eventually settle in to the optimum tune. This procedure is similar to how you do email tunes. You log some WOT, normal driving at different speeds, etc...then send the logs + a copy of your stock tune to someone. They tweak on the tune, send it back to you and you load it. You do more dirivng, logging and send that info back to the tuner. They tweak and send a new tune to you. I initially did some tunes by email with Bob at SVS and we did this process 4 times before we were both happy. Later, with more mods, I went on to a local CMR tuner and had the tune redone.
If you add another 5% but see no change in the STKR at WOT then you've hit the max the injectors can handle. IF it does change, then you are on the right track. To get it lower, and be able to handle more fuel, some SRT4 injectors may be in your future.
One key measurement you can't log unless you are on a dyno or have a wide band sensor in the exhaust stream behind the O2's is Air/Fuel ratio. That measurement is something you should get as you can go too far and see the A/F drop too much at WOT which is too rich.
Frankly, I think it's time to get on a dyno.
2006 300C SRT8