In my saga we have a Dimicatio nearby and I start to think about certain things.
For what I understand, for "counter-spell" a hermetic spell you need a spell with, at least, half the level of the original spell and it need to have some base to work. So, a Pillum of Fire (CrIg 20) could be stopped by a Creo Aquam of a shield of water or a Creo Terram for a wall Level 10, but no with a Perdo Corpus.
From this I have the next questions:
For confirmation: You use the final level? I mean, a Pillum of Fire but in Visual Range instead of Voice is more hard to defense against it even if the effect and power is the same?
Can you put additional levels in a spell even without changing its effects? I mean, I have Pillum of Fire Level 20, but I don't want it to do more damage or change some other parameter. Can I create a Pillum of Fire Level 40 and still have the benefits of, for example, Mastery Books?
If the second is true. You don't have any benefit for it? Only for confirmation.
There is a discussion going on in another thred about how this should work. According to the current wording of the rules two spells at different level are different spells, so a level 20 Pilum of Fire and a level 40 Pilum of Fire are two different spells. Different spells have different Spell Mastery Abilities.
There is a Spell Mastery special ability Adaptive Casting in the books which is supposed to let one Spell Mastery Ability work for several versions of the same spell at different levels, but due to how things are worded it is a bit of a mess exactly how that is supposed to work - or if it even can work without rewriting rules.
A Pilum of Fire learned and cast at level 40 instead of the normal level 20 would have few benefits except being harder to counterspell.
Note that you can't just increase the level of a Formulaic spell. If you want to cast it at a different level you need to invent it as a new spell with that level. (Partial exception to this is if you have Flexible Formulaic Magic.)