Likewise - tapering is not my strongest point. Tim, in your approach is week 4, the heaviest training load, and does it end with the last long run before the target marathon. If so how does your mileage fall in weeks 4,3,2 and 1. One of the problems is trying to maintain confidence during the taper, ie persuading yourself that you're not loosing fitness during the taper period of reduced training. It seems to me there are two ways of overcoming this, one is by reference to what you did on previous tapers and the other is to keep the intensity of previous speed/endurence sessions going, but reduce the volume. So for example a five mile lactic threshold run may be reduced down to 1-1.5 miles over the last three weeks or a 10-12 marathon paced run would be reduced to 2-3 miles, with last run done in race day kit and visualising the way you hope to feel at the start of the race. The Pfizinger book "Advanced marathoning" has a very good section on this.
Tapering for shorter races, requires a much shorter period (incisive huh!). If I race on a Sunday, for a 10k I will run 7-8 miles on each of Friday/Saturday at 70-75% MHR pace. For a 10 miler the Thursday run would be probably be 10-12 miles also at 70-75%. This compares with normal training pace of 78-82% MHR. Before last years Bath half marathon, I ran 17 miles on the Monday at 7:08 pace, rested on the Tuesday, ran 7 miles on Wed,Thurs, Fri all at 7:00 pace and 6m on Saturday at 7:30 pace. On the day of the race I felt great, and despite the gale force winds cruised through 10M in 61:15 (6:07 pace) before buckling slightly to finish in 80:31.
I tried exactly the same taper again three months later for Norwich Half to similar good effect. However during this intervening three months I did a weekly tempo run at LT pace, a weekly speed session of intervals at 5k pace and a weekly 13 miler at equivalent marathon pace. The effect of this was to improve steadily through this period, having the most fantastic weeks training four weeks before the Norwich race. During the last 4 weeks the speed of these sessions gradually slipped and I ended up running 81:40. This suggests that I peaked too early, and the max period of this type of training intensity for me is about 6-8 weeks.