This is a Rule that all of us overlook frequently. In politics, as in life, one can never achieve everything one sets out to achieve. Life and politics are a matter of give and take. Compromise is frequently a necessity.
This Rule provides that it is better to achieve 50% of what one hoped to achieve than to achieve nothing at all. Given that many of us are inspired by, became interested in or entered politics because of the strong leadership of Margaret Thatcher and Ronald Reagan, we often forget that there were occasions when both Lady Thatcher and President Reagan could not achieve everything they wanted and instead settled for "the good" instead of "the perfect". While both leaders had their total successes (such as the Falklands War or the collapse of communism), there were occasions when they had to settle for less than they had wanted at the outset (such as smaller tax cuts, less private involvement in the NHS and so on).
In the current context, this Rule would encourage conservatives to recognise that while some of David Cameron’s leadership may not be exactly what "true Thatcherites" or ideological purists might wish, in the majority of respects it is far better than what is on offer from our political opponents. Thus it is important not to make "the perfect" (the wish to apply pure Thatcherite values that some might wish) the enemy of "the good" (the espousal of broadly conservative themes by a new leader whose views are finally receiving a fair hearing from the media and the electorate).