I've never really liked Spell Adders, and Power Point Multipliers are, IME, often over-powered if you use any PP scheme other than the original RM2 one (in which case they tended to become necessary).
New, having read RMC and RMX, I've decided I rather like the option of granting PP = 10 + realm bonus/5 + PP/level. However, this clearly will make multipliers very powerful, so I've been looking for a way of reining them in a bit. I believe I have found a way.
I noticed that multipliers cost the same as an adder such that [multiple] = [add -1] (ie a x2 multiplier costs the same as a +3 adder), implying that these devices (were intended to) have similar utility. However, if we look at a 10th level spell user with 10PP, a x2 multiplier gives them +10PP, so each spell from a +3 adder would be worth about 3.33 PP, but I very seldom saw a spell user 'wasting' an adder slot on a spell that many levels below their own level, so that doesn't make a lot of sense. However, if the caster has 20PP, each adder slot is worth 6.67PP, which seems more like it (and as the caster's base PP/level goes up the multiplier gets better and better, likewise as the size of the device goes up).
From this I concluded that multipliers, especially large ones, were too good for their price compared to adders, unless the device was low powered and wielded by a character with few PP/level. As it's probably effectively impossible to make adders and multipliers universally balanced one with the other, and considering my dislike of adders, it seemed sensible to ditch adders, and then find some way of making multipliers lower powered, and to include some low-end multipliers to replace the lost +1 and +2 adders. In the end I came up with:
Power Point Adders
The power point adder is a device that increases the number of power points a character gains each level. Thus a +2 PP adder increases the PP/level that a stat of 90 grants from 1/level to 3/level. When calculating value, treat them as a spell adder one size bigger (so a +1/level PP adder costs the same as a +2 adder). There is also a +0.5/level adder that costs the same as a +1 adder. When converting adders and multipliers in published material, use the PP adder of equivalent price:
PP Adder Spell Adder PP Multiplier
+0.5 +1 --
+1 +2 x1
+2 +3 x2
+3 +4 x3
+4 +5 x4
+5 +6 x5
+6 +7 x6
Thoughts?