I can say as a practitioner of Japanese Sword Arts that technically there is very little difference in chopping something with a katana or a shoto or a tanto for that matter so a machete or a Khukuri shouldn't change the basic physics of the way you hold the sword or machete or the area of the blade you strike with. The grip is best if you hold the tsuka or handle lightly with your index and middle finger and more tightly or solidly with your ring finger and little finger. On the strike there is a slight, whip like, wrist action that should provide all the force that is necessary to slice your blade through the target provided that your blade is properly aligned to the target mass. Not sure if that makes sense. The area that you have indicated in grey on the Khukuri is indeed the 'sweet spot' for any blade in a blow. Make sure you relax your shoulders and breath when you strike and add a slight pivot or hip action to gain power, just remember the strength of your arm is not where your power actually comes from with the sword, it is mainly from lower down at the belly and hip. I know it is complicated but practice is a load of fun! I would love to see a scrapyard or swamprat khukuri as well.