A million chefs will have a million opinions on the proper way to hold a knife. Your style of doing this is your own, but should prevent your hand from ever curling under the knife, and should prevent diagonal cuts from accidentally creeping into your bâtonnets. I was trained to never touch the span of the blade with my index finger, unless doing fine crafting such as feathering with a paring knife. The knife hold that I teach uses a pinch with the thumb and the index knuckle on the bolster, just before the hilt. The pinch means you can use the blade as an extension of your arm and entire hand and can keep your wrist straight, without the hyper-extension of the index finger out. It’s all about preventing fatigue and increasing control. With this in mind, go forth and chop.