So the departing time should be as acurate as possible. And the exact time is the most accurate possible!
Yes, it should be as accurate as possible. So Spanish controllers should be picking up the phone and organising improvements with FMP/CFMU when they can. The flight can then depart early and CFMU has an accurate 'revised' CTOT. If your workload permits, why wouldn't you ? Or maybe your management can't read and understand the simple procedures in the CFMU Handbook ?
And I don't buy an excuse that a 'Royal Decree' won't let me comply with CFMU rules. If the 'Royal Decree' tells you to disregard what is published for the CFMU Region, then Eurocontrol and the European Parliament needs to start taking serious action against the Spanish Government or AENA. Perhaps withholding Route Charges for a period might be appropriate until the apparent Spanish non-compliance is dealt with ? Or Spanish Controllers can be proud of their profession and hold up the CFMU Handbook in front of their management and ask why they are not allowed to comply with the rules which are published ?
Let's also be clear about capacity in the system. There is approximately 30% more capacity available than the declared Traffic Volume. That is there to allow for emergencies and other contingencies, as well as to cater for the fact that traffic delivery is not an exact science. Even when a volume is regulated, the aircraft entering it can (simply by the rules of the system) be anywhere within a 15 minute 'slot' around the calculated time of sector entry and this is deemed acceptable by the system. Unregulated traffic should therefore be expected under the same criteria. Until we get precise 4D flight management, in the ground and the air, that's the way it has to be.