Hi Pete, all the above is pertinent, but to add further info....
When your clutch is getting towards the end of it's life, the wear reading becomes unreliable. The margin of error is just too great..... could be 50 percent, 70 percent, even 90 percent worn, but the wear reading will not measure this with any certainty. What does happen with a gradual but increasing regularity is the flashing gear indicator, which indicates gear not properly selected. This is indicating a slight slippage during engagement. Little known, but the SD reads slippage, and as the clutch reaches the end of its life, this is the more reliable guide to the state of play. I can't remember the numbers, but there's a bench-mark built into the SD and the reading measures the variance to this benchmark. Easily checked on an SD and the wear reading, in conjunction with the slippage reading will guide you to a real understanding of where your clutch is at.
Hope this helps..... Cheers, Em.