As I see it, the use of force against a burglar really comes down to whether there is an immediate threat to yourself and that your force is in direct defense/elimination of that threat. So the force has to be as much as necessary, but no more ( i.e. not excessive ).
In the case of the farmer, iirc, he was done because he shot the person in the back as the person was moving away from him … so basically the threat was dimishing rapidly rather than rising. If the burglar had been coming directly at him and the farmer under fear of his own life, then what might be a different outcome.
Another aspect that might come into play if you were to use your air pistol on a burglar could be whether you had any premiditation to use it. If you found someone in your house and in the course of doing so, found an object which you used as a weapon, then that could be considered differently to if you had kept a weapon under your pillow for the purposes of using against burglars ( if you get what i mean ).
That being said, i personally believe you have a right to defend yourself within the home.