My magus creates an item, lets call it the ring of splatting. Its a charged item with one charge and a total level of 40. He creates a lab text at lvl 40. This takes a single season.
Next season, impressed with the effects of the ring of splatting and wanting another one (he has used the one charge from the previous one), he uses his lab text to make another one.
Usually a lab text adds the level of the effect onto the lab total to create such a device. In this case 40.
He now has his original lab total, say 40, and the lab text, 40. Does this mean he creates a ring of splatting with 9 charges (+4 on lab total above what is required adding +8 charges). And if this is the case, does he then create a lab text for this effect that lets him create a ring of spatting with 9 charges instead of one?
As far as I could understand, having a labtext simply allowed you to not subtract the effects level before figuring out how many charges you could make in a season. So, if you're skilled enough to make a single charge on a lvl 40 item without a text, then you can make 9 with a text.
So, the second time around, you might make another labtext, but it would be (for game purposes) the same one, both producing "ring of splatting", and both requiring a labtotal of 40 to use.
The lab text is for "charged ring of splatting", the number of charges is irrelevant; it is only a function of your lab total and not of the item design. So your new lab text would be functionally identical to the first one (unless you decide to experiment, for example).
The lab text doesn't add to your lab total, it never actually says that, it just specifies that you can make whatever it is in one season if your lab total happens to exceed it. It specifically says the lab total is just worked out in the same was as when working without one, so...
Also, it has a specific addition for charged items near the bottom of that section (pg 102). That you just flat out produce an item with a number of charges = to 1/5th of the lab total, rounding up.
(+4 on lab total above what is required adding +8 charges). Unsure what this bit here is though.. where's this +4 from?.. you don't get similar spell bonuses for items unless you know the actual spell, you just have a lab text for the effect, which already has a bonus of its own. Arguably you could get a +1 for similar effect invested in and item.. but as it stands, there are no hard rules for adding charges to charged items.. purely by the rules, you'd technically just have to make a new seperate item, and thus not get the +1, but we just let people stick more charges in an existing item in our campaign. Also.. I guess you could argue that you don't get a +1 bonus because its you're not actually investing a new effect, just adding to an additional one.. but.. meh, who knows
In practice, with the limitation that your lab total needs to be large enough already, the net effect of lab texts is to add the spell level to the lab total. That's why you can invent the spell/enchant the device in one season (between the text and your own total, you reach twice the level) and why you can use your full lab total to determine how many charges are produced (instead of only counting the amount by which you exceed the spell level).
This way of seeing things was discussed when we pondered what would happen if a) you had multiple activities in one season (where the lab totals add up) such as enchanting multiple copies of the same item for which you have a lab text, and how to compute the number of charges you get for a time-limited charged item.