How will a stop watch help you if you’re driving a vehicle with a digital tachograph 
It matters not one bit what your stopwatch says it’s what’s on the tachograph that’s important, for instance there have been a few posts on here where drivers have had insufficient break simply because they calculated the break duration using the start and finish times when in fact the tachograph display is always right.
It’s the same with driving time, why use a stopwatch when the time you need is on the tachograph display, also given that the driving time will often click over to the next minutes after you’ve stopped, a stopwatch is likely to lead to problems on any occasions when you maximise your 4.5 hour driving time.
Sorry but I just don’t get this fascination with watches and various other gizmo’s when the important information is clearly displayed on the tachograph
A stopwatch may be of some help when you’re using an analogue tachograph but it’s totally pointless when using a digital tachograph in my opinion.