You have it right. Try explaining it with a different attribute, though, since Charisma doesn't often have augmentations to start with. Say you have Strength 4 and Muscle Replacement 2, making your augmented Strength 6. You would need to cast the Increase Strength spell at Force 6 (the current augmented value), and you could only benefit from two hits, bringing your augmented Strength to 8 (because, as you pointed out, the maximum augmentation is +4).