Angle 45 - the formula is easy, always use 45 and alter power
based on wind. This is only for 0 wind or wind towards the enemy.
Your shot must change completely when wind is against you.

Angle 45 guidelines:

1/2 screen: 1.45 bars
3/4 screen: 1.75 bars
1.0 screen: 2.15 bars

UP: lower power .1 for every 5 wind. E.G. 20 wind up, lower .4 bars.
a 1 screen shot goes from 2.15 to 2.75.

DOWN: add power .1 for every 6 wind. E.G. 25 wind down, add .4 bars.
a 1 screen shot goes from 2.15 to 2.55.

TOWARDS AND UP: reduce .1 power for every 3 wind, and then
reduce 1 or 2 mm more power, E.G. 10 wind up+forward, lower .4 bars.
a 1 screen shot goes from 2.15 to 1.75.

TOWARDS AND DOWN: Almost no change. Reduce .1 for 7+ wind. Reduce .2
bars for 17+ wind. E.G. 12 wind down+forward, lower .1 bars.
a 1 screen shot goes from 2.15 to 2.04.

DIRECTLY TOWARDS ENEMY: reduce .1 power for every 8 wind
E.G. 16 wind forward, lower .2 bars.
a 1 screen shot goes from 2.15 to 1.95.

Other info:

1. pixels count when measuring power.
1 mm of power is 5 pixels, or .05 bars. it's the smallest amount
that you can change your power when using slice. Two of this amount
equals .1 bars. There are 20 mm to 1 bar.
So if you know you must adjust .2, that's 4 of the smallest
amount you can control using slice. Also, if it helps to visualize it:

.2 = 1/5th of a bar
.35 = 1/3rd of a bar
.5 = half of a bar
.65 = 2/3rds of a bar
.8 = 4/5ths of a bar

2. I recommend slice for power control usually.
In some cases, drag may be better both for speed and accuracy -
you can drag to an exact pixel and sometimes a few pixels of power
(like 3 instead of 5) are needed to make the shot hit properly.
Most of the time slice is good enough. The key thing about fixed
angle formulas is to develop a feel with them, and I don't think
you can feel a dragshot.

3. When shooting half screen shots, the amount of adjustment won't
always be as large as the adjustment for a 1 screen shot.
The wind adjustment for 1/2 and 1 screen are close but not the same.

4. Watch for elevation, a small requires some careful changes in power.
It can help to visualize the path of the shot, imagine shooting a bit
past an enemy who is higher than you, try to see if their body
would 'block' a particular shot like a 1 screen shot. Alternarely, for
lower enemies, imagine something like a half screen shot and see if
the enemy lies at a 45 degree angle (roughly) to the place where
your visualized shot would pass you and go below your ground level.

5. Remember to play smart: watch wind, ditch the formula is there's
an easier shot or a different formula you're sure of.

Credits: Me for writing down info and screenshots. Untzremark for
testing, confirming, and the original idea.