Am I Giving Enough? The Only Safe Rule

Tim's sermon from Sunday reminded me of this line from Mere Christianity by C. S. Lewis:

I’m afraid the only safe rule is to give more than we can spare. In other words, if our expenditure on comforts, luxuries, amusements etc., is up to the standard common among those with the same income as our own, we are probably giving away way too little. If our charities do not at all pinch or hamper us, I should say they are too small. There ought to be things we should like to do and cannot do because our charities expenditure excludes them.