Credit to elguapo, from GBGL forums.

A lot of people have landmarks for halfscreen such as 70 2.0, etc, numbers around that.

Given the full power 1 screen angle, you can pretty accurately calculate what this shot will be -

sin(-1)(2sin(2A))/2=the new angle, at 2.0 power instead of 4.0 power

So, for example, cake is 81 full screen, so sin(162)=.3090,
sin(-1)(.6180)=38.2 or 141.8
angle 19 or 71 at 2 power will give you exactly halfscreen, provided that the fullscreen measurement is correct.

It also corrects the 70, 2 power measurement to a more accurate 71 :)

Turtle is 79 full screen, sin(158)=.374
sin(-1)(.748)=48.5 or 131.5
so angle 24 or 66 at 2 power will give halfscreen.

Of course you won't be calculating this in the heat of battle, so just calculate beforehand >.>

once again, the universal formula is