Theoretically you should need right around 25mm of extension tube to get you to 1:1. However, what you have right now might be a good compromise. It sounds like you've already used up the adjustment range of the ES-2, and adding more extension tube will only make that condition worse. 40mp is vastly more resolution than your film, so you can afford to use less than the entire sensor. You might even try slightly less overall extension if you have trouble obtaining perfect focus. Be sure to face the emulsion side of the film towards the camera, and then correct in post. Good luck! Can't wait to see your results.