Hi drano, there are a few sites that are very usefulbut you've probably already spent time at them....anyhow try here for the most comprehensive coverage of the topic
http://www.na.fs.fed.us/spfo/pubs/uf/utrmm/
all the info is downloadable for free; pdf's (of course) What's good is you can just take the area that relates to you and read that at your lesiure.
Personally I would caution when using Matheny and Clark's system not to get tied up by the number outcomes M&C clearly state that the number score at the end of their assessments should not be interpreted as quatifying the level of risk within an individual tree. I would also strongly advise that somewhere in your presentation you emphasise that there are no safe trees, they all contain risk just like the rest of our interactions with everything around us, its how much risk we are prepared to accept and what benefits that acceptance provides.........otherwise you end up removing nearly every tree you assess, or more realistically you have a system that merely permits the assessor to remove any tree they wish based on their individual preferences.
If you have the time and inclination go here
http://www.qtra.co.uk/cms/index.php?section=1
have a look through the site there are some downloads of the published paper on Quantified Tree Risk Assessment QTRA, its what I use and I have found it an excelent tool. The best aspect of this system for me is the reduced emphasis on tree defect and the overwhelming emphasis on target value. Anyway hope these are helpful links.