The simple answer to this is, as close as possible while feeling comfortable and anatomically smooth. Some have rather inflexible wrists and cant touch their thumbs together, others can. Wider than index fingers on the first section of knurling, not the middle knurl, the first patch of knurl going away from the center, is about as wide as I would go. Beyond this, your nearly doing a tate-style press.
Remember, any lift that is uncomfortable means either
1.) Severe strength imbalance
2.) Structurally, your body is under strain during the range of motion
3.) You have an impingement of some sort which is precluding you from doing the lift comfortably.
Aside from item 1 which can be corrected with a slow adaptation period, it is best to just follow a comfortable range of motion.
I'll give you an example. I am 5-10 and have a very long torso. My bench grip is pinkies on the rings and sometimes outside...and believe me my arms are perpendicular at the bottom with a grip that wide. . That is where I am most comfortable and also strongest.....and it is the shortest range of motion for me. If I come in with my grip, it becomes a heavy triceps exercise. One might argue "Well, just strengthen the shit out of your triceps and you can bench with a closer grip", yes and no, there is a point of diminishing returns where driving the load further cannot be made right by a stronger set of triceps....unless it is in the cards for me to have 26" arms. Hopefully that makes sense to you. You will have a certain hand/foot position that gives you the most leverage based on your own structure and lever arm pivot points.