When is a Lead’s Bowl Short?

There is a maxim addressed to leads at lawn bowls that says, “When you are down shot, don’t be short.” Other sources instruct leads more simply; just don’t bowl short at all. When the opposition has shot very close to the jack, it makes sense not to blockade your teammates’ path to removing it.

But what actually constitutes a short bowl?  Is a bowl one foot immediately in front of the jack short? How about two feet or three feet directly in front? Where does short start? Does it depend on how wide of the jack that bowl is?  It seems to me that a resting bowl should not be a problem if it is close enough to the jack that it can be easily promoted. In that case, wouldn’t 'short' depend upon the surface, because that would control how much a stationary bowl could be rolled up?

If the offending ‘short’ bowl is a meter or more to one side or the other of the jack, I can see that it is a problem, no matter what distance in front of the jack, but in this situation it is not so much the problem of an opponent potentially wicking off it (my side can use the same opportunity), it is simply that, if the jack gets moved backwards during the subsequent play, that bowl will become increasingly irrelevant to the count.