I figured how to add CRs according to the WotC method (which is not the same used in Grimtales):
CR uses a logarithmic scale. So, first bussiness is to convert CR to a lineal scale we'll call "power":
You calculate power by doing:
power(CR) = 2^(CR/2)
That is TWO raised to half the CR... and not the CR squared. When CR is an odd number, do the calculation for CR-1 and CR+1 (both of which are even numbers) and then average both results.
An exception would be CR 1, which has power equal to 1.
If CR is a fractional number, then the power is that same fractional number.
To convert power to an EL, you invert the operation:
EL(power) = log2(power) * 2
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So... you want an EL5 encounter... the power of an EL5 would be:
Since 5 is odd... we calculate for 4 and 6 and then average:
Power(4) = 2^(4/2) = 2^2 = 4
Power(6) = 2^(6/2) = 2^3 = 8
So, an EL5 has a power of (4+8)/2, which is 6.
Since CR1/2 creatures have a power of 1/2, I'd say 12 CR1/2 creatures make an EL5 encounter... of course, YMMV... they might make a very easy encounter nonetheless, depending on the specifics.
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Disclamer: I've found this system to work just fine when calculating stuff like the EL of CR7+CR7+CR8 when you compare it to the numbers suggested by WotC in published adventures. The bit about "the power of a fractional CR is the same fraction" I'm not too sure about, because that's just my best interpretation of what fractional CRs are supposed to mean... but I wasn't able to compare it with WotC's numbers.
