I would certainly be in favour of that, but doesn't answer the original question.
The gist of my original questions was: At what point does a lab text no longer provide its (full) benefit?
It comes down to how specific an enchantment lab text is with regards to the Shape and Material. Does a lab text always specify the full shape and material? For example, does a lab text for a lesser enchantment of Pilum of Fire into a staff apply to any staff? Or does the original lab text need to specify that this was a plain staff made of oak (even though oak provides no benetif) and cannot be replicated in a staff made of ash?
If the answer to that is that the lab text can be used on any staff, then a second question arises. Do additional bonuses provided by the S&M of new staff (e.g. from a ruby mounted on it) apply to the lab total? Note that all that does if perhaps save some time if this was a multi-season enchantment, or provide some additional charges for a charged item.
This might lead an enchanter to leave some of the S&M bonuses out when he crafts the initial item, so that the lab text is easier to reuse. Which makes sense, since he would want to be able to reuse that lab text making a wooden staff as hard as iron (Aegis of Unbreakable Wood) to be widely reusable to any wood item. So he might decide not to include any S&M bonus to make replicatation of that effect easier.