say if i'm out tahr hunting (got to do that again) and i'm standing above a big 13" bull(dreaming) and he's 20m below me do i shoot using a sight picture that i would use for a point blank shot?

Just jump on him, and break his back; he'll cushion your fall!

Seriously, though, I have no idea...

Very good discussion of shooting at targets above and below you in "Timeless Bowhunting," by Roy Marlow.  In short, there's a tendency to shoot too high whether the target is above or below you for a number of reasons.  The animals look farther away than they are.  The horizontal distance to the target is less than the angled distance and thus gravity has less time to work on the arrow, etc.  A sight shooter would shoot the horizontal distance to the animal, not the vertical.  So if the animal was 5 meters out and 20 meters down, the sight shooter would shoot the animal as if it were 5 meters away.  To answer your question, yes, you would pretty much be holding dead on.  Remember to bend at the waist though to keep your draw length from shortening.  But don't wait until you have a tahr standing under you.  Try to practice that shot now, you lucky devil.

lay that bow over at least it helps me.

bend at the waist and "aim" lower than usual

Byron Ferguson says to bend at the knee , front leg knee bent on down hill shots and back leg knee on up hill shots . Ive been working on this my self , if your like me setting in a deer stand I have to bend at the waist because I dont stand up to take the shot .

Mike

It does depend on the angle of the shot. If it real acute angle I was always told if downhill take 5 yards off the shot and if uphill add 5 yards...

now I know about bending etc, but it never worked as good to me as just  +- 5 yards did for me.

Bill

IMO If the angle is over 45 degrees, you must aim a little low, whether it's uphill or downhill.
    This would apply within the effective range of any projectile.
                              Lin

Whats a Tahr?  
I am in agreement with Orion.  Baisicaly you can shoot well as much as you can bend at the waist and keep the "magic T".  Yoga practice any one?   :D  
D-

Orion's got it, bend at the waist, but when thinking about that PICK A SPOT.  I believe more people miss thinking about other aspects of their shot instead of focusing in on a single small spot.  Trust your form and practice so when you get that shot it is natural and all you have to do is pick a spot, release, follow threw, and the arrow will hit right where you are looking.