Since your scale (like PageRank) is a logarithmic scale, it makes it hard to judge the distance between 2 scores.  Can you give me a rule of thumb.  For PageRank,  each jump is an exponential jump - so that a PR6 is perhaps 10 times stronger than a PR5. What is the log base that SEOMoz uses.   Should I assume that a 60 is 10 times stronger than a 50? This is important when it comes to measuring progress because growth is going to get more difficult as you move up the scale and I need to communicate the distance between our current Authority score and our goal. Thank You!