I took another look at this. I think the metric you want to compare is not the area within the cone, but the chord length (straight line) or arc length (curved line) at the end of the cone. This tells you how wide an area the cone will hit at maximum range. This is what we currently have:
Radius
Arc
Chord Length
Arc Length
Torrent/Umbral Torrent
80
30
41.41
41.89
Tenebrous Tentacles
40
40
27.36
27.93
Night Fall
60
20
20.84
20.94
With respect to Torrent and Umbral Torrent, if you reduce the radius from 80 to 60 feet without increasing the arc, you're nerfing the power by reducing the width from 41 to 31 feet. To maintain roughly the same width at a 60 foot radius, you need to increase the arc to 40 degrees. Here's what you end up with at various combinations of radius/arc:
Radius
Arc
Chord Length
Arc Length
40
50
33.81
34.91
40
60
40.00
41.89
50
40
34.20
34.91
50
45
38.27
39.27
50
50
42.26
43.63
60
30
31.06
31.42
60
35
36.09
36.65
60
40
41.04
41.89
My recommendations would be:
Torrent/Umbral Torrent: Leave alone OR reduce radius to 60 feet and increase arc to 40 degrees
Tenebrous Tentacles: Increase radius to 50 feet and increase arc to 45 or 50 degrees OR increase radius to 60 feet and leave arc at 40 degrees
Night Fall: Maintain radius at 60 feet, increase arc to 40 degrees