Mine's a double

How do you know which values to double when doing Born-Haber Cycles?

Simple really.

In a compound like CaCl2, you always have to deal with 2 chlorines so any value which refers to chlorine will need to be doubled to make it refer to 2 chlorines.

In this case that will be the enthalpy of  atomisation

1/2 Cl2 (g) --> Cl (g) needs to be doubled

and

enthalpy of  electron affinity

Cl + e-  --> Cl- needs to be doubled.

The possibility is in something like Na2O, now you are dealing with 2 sodiums so you would need to double the sodium values, i.e.

enthalpy of  atomisation

Na (s) --> Na(g) needs to be doubled

and

enthalpy of  ionisation (energy)

Na   --> Na+ + e- needs to be doubled

It is worth being careful though as every now and again they pre-double some things for you in the table, so have a read and double only if it refers to one Na (or whatever) and you need it to be 2.

It's at time like this that I wish blogspot did superscript and subscript.