I ride 2 fingers. that's just how i learned it. but now i ride with my middle finger doing the pull and my indexes just hover but barely touch the lever. you get a bit of a better pull with the middle finger but i think that's just for my lever set up. they are long and very close to the grips. if you use one finger (middle or index) you can modulate well, if you use two, you're more likely to lockup.
If you can get adequate braking power, use just one finger. I find that with my levers positioned correctly I get excellent stopping power with only one finger on my 45+ pound dh bike. It really depends what works for you.
I recommend fiddling with your lever location until you find a place that works for you for 1 finger. unless you have an even heavier bike than mine, significantly weaker forearm/hand strength than me (and I am by no means strong), or a fairly terrible set of brakes, there should be a location for your lever that allows you to comfortably brake with 1 finger.
kk yea i weighed my bike the other day and found it was 40 even. i've installed new pedals so it's under 40 now. but yeah i'll look into repositioning the levers. the rear brake is dying for a bleed and the front one wouldn't hurt to fix too. i have a feeling my levers are a bit long but i'll see if moving them in helps.
yeah, ideally you would have it so the pointer finger is right at the edge of the lever, and the brake is at the same angle your forearm is when standing up out of the saddle.