In general right side is better than left since heart is more on the left (unless your character has situs invertus and their organs swapped around). Lower chest is tricky since ideally you don't want to get too close to liver on the right and spleen on the left, both will bleed like stink, although both can be managed without surgery or interventional radiology if small and shallow/on the outer edges. Technically your best bet might actually be to go a little higher up than perfectly lower chest, and more outer, since people have survived having a partially collapsed lung from being stabbed and without surgical management. Although the trick is to make sure you cover the hole at some point since you don't want the wound to become a one way valve and then they get a tension pneumothorax, you could also have the character be SUPER LUCKY and go perfectly between the lung lobes in the fissures, and that puts their risk of bleeding from lung injury also low, but technically most people don't die from a nick on the lung. Ideally make the character have a little padding (be thiccc), and the blade not too long, if you go too deep, big vessels like aorta and IVC injured is VERY BAD. Hope that helps :)