Most times this can be ignored.
The ISA DEV value has a flat line (unable to recall what the limits are) in performance graphs between which there is nil impact on profile. Outside these, there is a tapered adjustment.
During a performance seminar once attended, the answer given was that in moderate latitudes this value can comfortably remain void. Only when operating in extreme warm or cold air masses should these be considered for input.
By memory, but please do not quote me on this, the values were ISA-10 to ISA+15.
Guidance given during the seminar was to take TOD ISA, and FL100 ISA as this represents more then half your profile accurately, also as many airlines do not provide ISA deviation at lower levels then this (current outfit uses LIDO and gives 300/200/100). It was specifically advised NOT to use ISA on ground for creating an average from TOD-GND, as very low level temperatures have much more influence from ground then air even 2000' AGL.