I just keep an eye on my average speed but I don't worry about it. I don't want to flog myself trying to keep it up. I keep an eye on my current speed over a few hills to see how I'm going. Using the average speed to pace yourself means that by the time you know you're slowing down, it's probably too late to do anything about it.
The main thing is to pace myself though. If I'm unfit and slow, it's too late to do anything about it on the day. I just try to pace myself and keep an eye on my speed. If I slow down a lot, I'll stop for a feed and drink and see if my speed picks up again, unless I'm near the end and know that it's just tiredness, then I'll just try a bit harder if I can.
Once you've done about 12 hours, you have to slow down quite a bit for some time before it shows on your average, That's when I tend to relax a bit, especially if I'm going well. Just get through the night and lose as little as possible. Then when it gets light, I try and build up my average again, rememberring that the best way to do that is to keep riding a little bit faster all the time instead of blasting down the road for an hour then collapsing in a heap and losing all that you gain. As the average speed increases, i work out what I can achieve and set targets in my head which help me stay focussed instead of just rolling out the last few hours. On the other hand, I still try to look after myself so that I can keep going and stop to get food and drink if I think it will help. A few hours of hard effort when hungry or thirsty can take hours to recover from by riding slowly. Better to stop a few minutes and keep going fast all the time and feeling good (or at least not awful)
I know that 420 miles is about 18mph and that on a good day I can do 21mph for the first 100 miles. So as long as my average is between 19 and 21, I know I'm in with a chance of getting 410 miles. If my speed is 18, I'l not let it bother me early on and see if it improves. Either it will and I'll be in with a chance of 410, or it won't and I wouldn't have done it anyway.
I think I'll be trying to average 19mph for the first 100 (no doubt I'll try for 20 at first then realise that I'd better ease off if I'm going to keep it up, I'll have to wait and see) this year, but will have a better idea after riding the Seething 600 with my 24 hour TT set up.
I don't account for whichever circuit I'm riding. I prefer the Quina Brook, but it probably is slower.