You ask a good question, Chris. Take a look at the curve in the graph I posted above. It has a sigmoidal shape, roughly. The first mutations have little effect on protein activity. This buffering effect is pronounced for wild type proteins. (I am excluding active site mutations.) Then there is an exponential decline in activity as you noted. This is followed by a very low level of activity for an extended period of increasing mutation. Doug did not want to measure where buffering was taking place, neither did he want to measure during exponential decline. He wanted to measure at the threshold of enzyme activity, because he was trying to quantify the number of changes to go from no function to function. This is the transition boundary that is of interest to evolutionary biologists. Therefore he used a weakened enzyme to represent that initial emerging enzyme, and he determined what proportion of mutations would reduce that enzyme to lack of threshold function. That was the proportion he was seeking—what proportion of folds were capable of carrying out a minimal enzyme function.