Trying to boresight a handgun is an exercise in futility. There's not enough barrel length or sight radius to get any kind of meaningful alignment done that way.

Just mark the slide at the center of your old sights, push/knock them out, and install the new ones centered on the marks. Then shoot it at whatever distance you want to zero (I'd recommend at least 15 yards)...preferably with your hands (not the barrel) on a bag for stability, and push right or left to fine-tune as necessary.