Personally, I don't see any problems with setting Withering damage as the square root of your HD. It's an extremely simple calculation that can be done mentally (at least until 1000+ HD), and if you're playing using Immortals Handbook rules, you're probably going to be fairly proficient at math anyway.
Well, I'm going to go compare the damage values of sqrt(HD), 1/20 HD, and 1/10 HD. I'll use Perfect Withering (Constitution) Beam as a basis for damage, since that seems to be the most brick-shittingly powerful of the effects, what with its reducing maximum hp per HD and all.
Perfect Divine Beam, Perfect Force Beam, Perfect Energy Beam, Perfect Aligned Beam, Perfect Bane Beam = 1 damage die per HD
(All damage values are averages: 2 per HD for divine, 2.5 per HD for force, 3.5 per HD for energy, 4.5 per HD for aligned, 6.5 per HD for bane)
30 HD... 60 divine, 75 force, 105 energy, 135 aligned, 195 bane
40 HD... 80 divine, 100 force, 140 energy, 180 aligned, 260 bane
60 HD... 120 divine, 150 force, 210 energy, 270 aligned, 390 bane
80 HD... 160 divine, 200 force, 280 energy, 360 aligned, 520 bane
120 HD... 240 divine, 300 force, 320 energy, 540 aligned, 780 bane
160 HD... 320 divine, 400 force, 560 energy, 720 aligned, 1040 bane
240 HD... 480 divine, 600 force, 640 energy, 1080 aligned, 1560 bane
320 HD... 720 divine, 800 force, 1120 energy, 1440 aligned, 2080 bane
Perfect Withering (Constitution) Beam = sqrt(HD)
30 HD... 5 Constitution drain = -2.5 Constitution modifier × 30 HD = -75 maximum hp (-150 in home plane due to ×2 hp multiplier)
40 HD... 6 Constitution drain = -3 Constitution modifier × 40 HD = -120 maximum hp (-240 in home plane due to ×2 hp multiplier)
60 HD... 7 Constitution drain = -3.5 Constitution modifier × 60 HD = -210 maximum hp (-420 in home plane due to ×2 hp multiplier)
80 HD... 8 Constitution drain = -4 Constitution modifier × 80 HD = -320 maximum hp (-640 in home plane due to ×2 hp multiplier)
120 HD... 10 Constitution drain = -5 Constitution modifier × 120 HD × 2 hp multiplier = -1200 maximum hp
160 HD... 12 Constitution drain = -6 Constitution modifier × 160 HD × 2 hp multiplier = -1920 maximum hp
240 HD... 15 Constitution drain = -7.5 Constitution modifier × 240 HD × 2 hp multiplier = -3600 maximum hp
Perfect Withering (Constitution) Beam = 1/20 HD
30 HD... 1 Constitution drain = -0.5 Constitution modifier × 30 HD = -15 maximum hp (-30 in home plane due to ×2 hp multiplier)
40 HD... 2 Constitution drain = -1 Constitution modifier × 40 HD = -40 maximum hp (-80 in home plane due to ×2 hp multiplier)
60 HD... 3 Constitution drain = -1.5 Constitution modifier × 60 HD = -90 maximum hp (-180 in home plane due to ×2 hp multiplier)
80 HD... 4 Constitution drain = -2 Constitution modifier × 80 HD = -160 maximum hp (-320 in home plane due to ×2 hp multiplier)
120 HD... 6 Constitution drain = -3 Constitution modifier × 120 HD × 2 hp multiplier = -720 maximum hp
160 HD... 8 Constitution drain = -4 Constitution modifier × 160 HD × 2 hp multiplier = -1280 maximum hp
240 HD... 12 Constitution drain = -6 Constitution modifier × 240 HD × 2 hp multiplier = -2880 maximum hp
Perfect Withering Beam (Constitution) = 1/10 HD
30 HD... 3 Constitution drain = -1.5 Constitution modifier × 30 HD = -45 maximum hp (-90 in home plane due to ×2 hp multiplier)
40 HD... 4 Constitution drain = -2 Constitution modifier × 40 HD = -80 maximum hp (-160 in home plane due to ×2 hp multiplier)
60 HD... 6 Constitution drain = -3 Constitution modifier × 60 HD = -180 maximum hp (-360 in home plane due to ×2 hp multiplier)
80 HD... 8 Constitution drain = -4 Constitution modifier × 80 HD = -320 maximum hp (-640 in home plane due to ×2 hp multiplier)
120 HD... 12 Constitution drain = -6 Constitution modifier × 120 HD × 2 hp multiplier = -1440 maximum hp
160 HD... 16 Constitution drain = -8 Constitution modifier × 160 HD × 2 hp multiplier = -2560 maximum hp
240 HD... 24 Constitution drain = -12 Constitution modifier × 240 HD × 2 hp multiplier = -5760 maximum hp
As you can see, Withering [Effect] is still powerful once you get to Elder One-level and above due to that flat ×2 hp multiplier. The square root method seems to be the most balanced up until that point, but past that it seems that 1/20 seems more balanced