On the other hand- if you want a level 35 version of a spell and you have a level 40 version as a lab text, and have a level 40 lab total, you can spend 1 season with the lab text and learn it at the 40 point version and get the +8 similar spell bonus, dropping the time to invent the spell from 7 seasons (lab total 5 over) to 3(lab total 13 over), so it isn't exactly useless.
The only real guidelines are "a special version of a formulaic spell must be invented" and the fact that longevity rituals are always designed for a particular target. Longevity ritual lab texts are only usable in a fashion that reads more like casting tablets- allowing the ritual to be repeated without taking a season. On the other hand under lab texts it does indicate that the number of charges in a charged item can be varied, and correlates to the current lab score instead of being based o the lab text results.
So the text seems to favor the idea that whom a custom designed spell is designed for cannot be changed- though on the other hand there is also no possibility for a standardized longevity ritual which inflicts warping when cast, or using a longevity ritual on someone else with those effects, so the correlation is not a great match.
And that's bad?
Say someone wants a sight range Incantation of Lightning and they have the lab text and a level 45 lab total. They either take 8 seasons to invent it from scratch, or, as mentioned, learn the lab text version, then, with the +7 similar spell bonus, finish the spell in 4 seasons (if they had a 46 lab total it would be 3 seasons.)
That's a huge investment in seasons for a small benefit. Who would bother spending 4 seasons just to have the option of sight range when needed.
The example would be a similar dynamic for any spell near pushing the wizards limits.
Even with weaker spells, the rules push dissuade people from adjusting spells. A magi could learn 3 level 15 spells from a lab text, or design one level 20 spell because they wanted a different range or targets.
Who would sacrifice learning 2 more spells, for that small amount of greater options the different range or target gave?
Here it really depends on the spells whether there is reason to invent the single level 20 or the three level 15. Maybe touch range (for instance) is just not acceptable for the purpose they need the spell for. Personally I often see low level lab texts as useful for filling in when you have a little more lab total than you need for the thing you really went into the lab for so I probably wouldn't just sit there and learn all the same Technique Form level 5 & 10 spells in the library unless I had a reason I needed one of those spells for a particular thing I knew was coming up.
I think if you want to make a lab text more useful for creating new spells based on a recorded spell the easiest approach would be for the lab text to provide a spell familiarity bonus according to the spell it represents, essentially saving you the season of learning it (or perhaps allowing it to be used when the spell in the text is too massive for your lab ability- depending on how your house rule handled that. It would still need to fit the normal criteria for related spells however.
So (under this house rule, most expansive version) if you have a lab total of 35 and want to learn a level 35 version of a level 40 spell for which you have a lab text then the text would provide a +8 bonus to your lab, allowing you to invent it in 5 seasons.
Not necessarily, and the point I tried to make was not in the sentence you quoted. Your proposal makes the existing similar spells bonus for spell invention meaningless, and thus the entire ruleset becomes inconsistent. That's bad. You could certainly make the house rule, but it is not a simple one. It has knock-on effects on how one should invent new spells when a similar one is known.
Actually, the case when they do not have the lab text is far more interesting. With your house rule, you should then invent a touch range version (L30) in two seasons, and then just vary that, it is two range changes adding five levels each, and you can invent your sight range version in the third season, using your lab notes. So you have cut the invention time from eight seasons to three, when no similar spell existed à priori. That's a major power boost.
I can see arguments as to why having a lab text for a spell be somewhat useful, but giving the entire similar spell bonus seems excessive, to me. maybe half the similar spell bonus, so having a lab text of a similar spell with a different r/t/d gives some bonus, but not as much as actually knowing such a spell.
Depending on game and logic either could make sense, and either is a bonus above what RAW would give.