If you don't fancy the maths and want to work out the percentage increase in your horses ability from one year to the next, just pop in the results of two training runs at the two different ages. Remember that both training runs should be on identical going and over identical distances.
For example, if the TT as a 2YO was over 2400 and on soft going, then, the TT at three must also be over 2400 and also on soft going.The percentage fitness however, can be different, one training run could be at 77% and the other at 93% for example. The programme calculates the variance in fitness and factors this into the result.
It is recommended that more than one comparison is carried out to verify your result as the training random factor can give a false impression if just a single comparison is made. I recommend 10 or more comparisions and then take the average.
Experience however, will let you form your own opinion of what is considered good or otherwise type age change.....