The CAA examiners are perfectly entitled to state that your batteries are dead , or your super slide rule for working out drift has fallen on the floor . Track correction and working out headings and drift etc etc are requirements of the cpl skills test and you should know how to do it mentally . If you have done a cpl course and cannot do it then perhaps you should be having a word with whoever instructed you . By all means use your CX2 but the examiner cannot be held to appeal for simulating a potential real situation , yeh you may have spare batteries but sposing you drop one under the seat whilst fitting it ??? You are travelling at 2 miles per minute you have been diverted elsewhere its a damn sight quicker to work out your heading with clock code etc etc than it is to piss about with a cx2 .
After all when the examiner closes the fuel cock whilst dloing a steep turn you arent going to complain to the CAA are you , its shtuff you should know how to deal with .
I am the thickest man in the world , still no idea how I got through the atpls etc etc but even I could work out a heading in my head when being diverted .
Just my personal honest opinion , do whatever suits you personally.
I have CX2 but have never really used it even in cpl ,