I just did go trough the Blessing - Meditation (RoP:D 51) guidelines and found a very interesting effect that my holy magus most likely want to invent as ritual for use with his holy magic.
Personal I would sort it into Muto Corpus for humans to invent a Holy Magic spell based on this guideline. (Make a body resitant to damage is the closest posible Corpus guideline and thats Muto)
Then we have +4 Year duration + 1 Touch +0 Circle if Base 4 works that would be lvl 25
But here is where I'm unsure what do magnitude of the effect means exactly?
If it means the Base must be in level 5 steps then this will be mostly uninteresting but if it means that a lvl 25 like above give +4 or +5 bonus to the aging rolls it would be fantastic!
(also be careful there is a errata that changed the text of this guideline but the example spell never got fixed)
There are similar guidelines elsewhere with the same problem. There are examples that go with them that use Base 5 specifically. Of course, those are only examples. But why use Base 5 or even Base 4 if you could use Base 1? Base 1 for +1 would give level 10 in total if that is accepted. I don't think I've ever seen an example doing that; all I've seen use maximum levels for given magnitudes (e.g. 5 instead of 4 for 1st, or 10 instead of 6 for 2nd).
However, if your question is just about using spell level versus using base level, guidelines refer to base levels. Adjustments are applied afterward.
the lvl 25 ritual is interesting because it is the highest level considered Constant Effect, Low Power and so I don't need to personalize the spell to a target because of warping.
Edit: OK thanks so this means a base of 5 gives only 1 to the aging rolls and not 5 as I first suspected (Base 10 = +2 and not +6). What make this guideline not that interesting for a Magus who can make a Longevity ritual.