As Mary says, "unable" is good. As for the miscalculation, we mess up just as much as anyone, just say, "apologies sir/madam, temps are much higher than forecast" (we get that a lot in high level) etc. If you're polite then the person on the other end should be too. Then again we all have our bad days I guess. The safety of the aircraft is your responsibility, separation of traffic is ours. If you can't comply with a method of separation then it's a pain but get in early what you need and it's rarely an issue.