Pre-race warm up: how much is enough?

Here's the thing. I always swim my best times at the end of practice or at the end of a set. I don't deliberately hold out (maybe in a set but not in a practice as a whole). If I am doing a set of 10 x 50 free sprint, I always descend the set and usually in 1 second intervals. So if I did the final 50 in 27 the previous one would have been in 28 and the one before in 29, etc. In fact it is almost as if I have to go through the whole progression to get to my best times. I find it very difficult to just dive in and do a good time. What should I do at a meet? I don't know what the situation is going to be with warm up pools etc. I know I would want to do at least a 500m slow warm up and then, at least, 5 - 7 fast 50's getting my time down as low as possible (without burning myself out) on the last few. I don't know if this is going to be possible though. What does everyone else do for a race day warm up? Syd