What is the most important thing...

[nyslowhand]

This angular compensation is not something that the average bowhunter needs to clutter their brain with while at full draw.

There are two exceprions.

If the deer is less than 10 yards, the difference in horiz & line of sight distances could be ~3yds.

Probably not a shot I'd make anyways. Shooting stright down at a deer. You've lost 1/2 your kill zone & single lung shot is your best hope.

Second exception is for the eagle bowhunters than are more than 25' in the air.

The difference in the two distances becomes more pronounced from those heights.

For the average bowhunter whose platform is generally 15-20' high, would you compensate for a 1-2yd difference if you knew that in advance? Probably not, unless you shot 1" groups at all your sighted yardages.

Like WNYbuckhunter said, there are other things you should be concentrating on when in the heat of the moment.

Don't mess up an opportunity by trying to out-think or over analyze your shot!!

[SteveB]

With today's bows a couple yards shouldn't make a difference . I think the problem is that folks forget to bend at the waste and drop their bow arm . This will certainly cause a miss .

100% on this one. If a yard or 2 means you miss the deer, the bow must be shooting like 90fps - I get more then that from my recurve.

[VenaticOutdoors]

Bending at the waste is important.

Also, if you want to get real serious, that diagram you provided leaves out gravity with a downward acceleration of 9.8 m/s2.  The higher you are, the more the downward angle, the faster your arrow is going to fly.  Suddenly, 13 yards becomes less than 12. 

Like everyone said though, if you focus on the basics like bending at the waste and a solid anchor point, then everything else will become obsolete.

[Buckstopshere]

A good rule of thumb when you are in a hunting situation, at full draw in a tree stand is to AIM LOW. Shoot for the deer's heart, right up the front leg. KISS. Keep it simple... don't be doing trig when it's time to put your arrow through a deer's chest.

But the most important thing to me is to make sure your broadheads are scary sharp.

Best of Luck out there everyone!