I use single finger, it didn’t take much effort or thought to re-train myself really, 2 finger braking seems to result in jabby, skiddy braking and forearm cramp (so called “arm pump”) single finger braking is smoother and more controlled, and you learn to spot braking points better…
I’m an index finger user, I’ve noticed a few people use their middle finger (including Steve Jones), can’t really see the difference but could be worth a go..
As for lever positioning, I angle them such that my finger is in line with my forearm when in my “normal” riding position, this varies slightly from bike to bike of course as I have a different position for DH and XC riding but the principle is simple, you don’t want to over extend the various tendons in your arm/hands and you don’t want to twist your wrist and potentially break/sprain it…
I see way too many bikes with the levers near vertical, looks stupid and is probably quite dangerous…