The formula we should use (in the European area) for departures with no CTOT is:
Earliest airborne time = (EOBT - 15 + taxy time) to EOBT + 15 + taxy time.
...
At my airport, for example, this means airborne between EOBT - 5 until EOBT + 25 for a departure without a CTOT. So start-up clearances are timed to allow for this, and current traffic conditions.
Your formula and policy is perfectly logical, but unnecessary. While a CTOT applies to airborne time, and requires the cooperation of ATC to get the aircraft in the air in the correct CTOT-5 to CTOT+10 window, the -15/+15 restriction on EOBT is exactly that, and it's permissible to get off blocks at EOBT-15 and taxi quickly, or at EOBT+15 and taxi slowly.
I get the impression that the latter is
much less critical to the smooth running of the system than adherence to slot times (which tends to be no better than 90% anyway). Nevertheless, the law requires ATC to ensure that "(c) flights not adhering to their estimated off blocks time, taking into account the established time tolerance, are not given take-off clearance;"
I think this causes unnecessary grief. As reportyourlevel indicates, while a DLA is easy, bring forward an EOBT is complex, requiring a cancellation and refiling of the FPL. Among some operators, this leads to a systematic error in estimating EOBT, as it's easier to file early and delay than the reverse.