Ok ill just add my bit to help make this clear cause im not sure it is.
Assumed Temp is something that operationaly in an airline you get from tables produced for your airline. These take into account the runway length, climb out clearance etc. Not only do you get one per runway but also one per intersection you could use to enter the runway. Also you have diff. corrected tables for different flap settings, wet runway, bleeds on/off etc. Into these tables you put the weight of the aircraft and read off the assumed temp (and also V1,Vr,V2 by the way).
The assumed temp is then input to the FMC via the CDU and the FMC then calculates the reduced N1 for the given Assumed temp (ambient is entered also). So as you can see all the other factors have been taken into account by the company designing and producing the Tables.
Note: If your lucky enough and your company has paid for the extra these tables may be replaced by the Boeing Laptop Tool (BLT) or as it's now known OPT. Into this laptop you select rnw in use, enter the weather conditions, aircraft weight, flap setting and even any MEL/CDL issues with the aircraft and it gives you assumed temp,N1(so you can check it),V1,Vr,V2.