Who or what told you out of regulation?
If they’ve been analyised by the agency then they should at least tell you where you went wrong.
Is it using the tacho itself, 2 main makes and differ slightly how they work or is it keeping a check on hours you’re having problems with?
Personally I think you prioritised the correct things of not trying to hit anything, screw up, over getting the tacho right. 
I do remember my first days out on multi drop trying to keep a track of hours, and that was before they threw WTD on top.
I think as others have said keep it simple,
Maybe when you start work, work out what time it will be in 6 hours and make sure you get a break before then that will sort out keeping an eye of WTD if you’re not doing many hours driving.
If you’re doing a lot of driving then you will get the tacho flashing a warning at you at 4:15, but try and plan ahead of that as it’s not always possible to find somewhere to stop in 15 minutes.
As somebody said if you’re driving a reasonably specced modern truck, you can get a display on the dashboard with all your drive time and breaks.