You need to figure this out in dry fire. I've struggled with this exactly problem through many grip changes and I've found a process that I'm pretty happy with for now.
First, do "blind draws" (draw with eyes close) with your normal grip. Open your eyes and notice where the front sight is. The reason for doing it with eyes closed is because this will help you understand your true index. If you look at the gun as you draw, you'll have the tendency to put a slight effort to align the sights as you complete the draw, which you don't want to have to do.
You'll likely find that you're way off, or a little off, or whatever. In order to adjust how the front sight indexes when you do a blind draw, you're going to have to adjust how much your fingers wrap around the grip with each hand. I like to use the trigger guard as an index for understanding where my fingers need to be. For example, does your right hand need to hook very far around so that your first knuckles point directly along the line of the trigger guard? Or maybe not as much so it lines up in between your first and second knuckles? Keep doing this and making adjustments until you start drawing blind and suddenly you open your eyes and the fiber is right where it needs to be.
Now drill those hand positions into your muscle memory. This is going to be done by slowing down the sections of the draw where each hand initiates contact with the gun. Make sure you have the correct "feel" when grabbing with your strong hand, and also find the correct feel when adding your weak hand. Then speed it up and make sure you're still hitting the right hand positions.
As a supplement, I'd suggest "testing" your new grip by practicing wide transitions (at least 90 degrees) in dry fire. Establish your grip and aim at one target, then transition as fast as you can using target focus to the second target. Did your sights stay perfectly aligned even though you swung your hands very fast to the other target? Go back and forth a lot so you look like a retard swinging a gun around. Make sure your grip keeps the sight in the notch even when you transition violently.
Unfortunately it's not as simple as that, because things change once you actually introduce recoil. Now that you have the "feel" of hand position in the grip, you need to get the feel for what pressure to use with each hand. I'd suggest doing this by firing single shots at a target and really paying attention to how your sight tracks. Adjust your grip, especially forward/downward torque with your weak hand until the sight returns exactly flat and doesn't dip too far or stay a little too high. Also play with inward pressure applied by the palms of your hands (think about crushing the top of your grip in between your palms). Bill drills are a good supplement to make sure your sight tracks consistently.
Once you've played around with grip pressure, you need to again make that exact grip pressure natural when you draw the gun.
It's a lot of work but by doing all this stuff in dry fire and practice you'll take so much effort out of the actual shooting because your muscle memory will be working for you instead of against you.
Edit: Why would you buy a SIRT when you have the best practice tool available already (hint: it's your actual glock).
