Global Penetration CrVi gen
R: personal, D: concentration, T: group
All effects in the device gain penetration of 2(level - 20). This effect may be worthwhile in an invested device with multiple offensive effects or which has multiple effects and may be used as a melee weapon.
base pen/2, +2 group, +1 concentration, +1 device maintains concentration
I'm not seeing a guideline for this effect under muto vim in the main book nor a relevant comment there. Nor anything directly relevant in the linked thread. I know most metamagic is MuVi but in this case neither the effect descriptions nor their RDT are being changed - they are just getting added power for penetration so I stick by CrVi maybe with a Mu requisite but I don't think it's necessary.
There's nothing wrong with inventing our own guidelines - in some ways I think it's unfortunate that each new supplement solidifies what is possible and limits the options for just making stuff up (unless they happen to agree with my ideas, then it's OK ). In any case the logic behind my numbers for this is pretty simple - we know what penetration costs in an item, we know what a group target costs, we know what maintained concentration costs et voila!
But even if you refuse the "penetration boost", you can manage to gain it by another way:
Example : your labtotal is 40 in CrIg, 50 in MuVi.
Rather than putting an effect level 20 in Creo Ignem with 0 penetration at voice, you choose to put an effect level 15 at touch and another effect MutoVim "wizard's reach" like. Obviously it has to be 20 and the change is up to 20+5 so your effect needs to be 25 in MuVi
You gain 5x2 penetration (15->20) and you have the same effect, needing a double effect rather than a simple.
In this example: MuVi 25 + CrIg 20 for a penetration of 10
In the Magi of Hermes way, with a level 20, if we want to boost his penetration by 5, we need a Muto Vim level 20 at least, but also at least 20+magnitude of change, = 25.
So a MuVi 25 + CrIg 20 for a penetration of 5
So, frankly, if we stick with RAW, in item, it's better to use the normal Muto Vim (penetration is 2:1). In spell, it may be different since the penetration is 1:1.
Problem is, the MuVi has no penetration itself so in a spear shaped item you can't stab a creature with might. It's even worse if you have a weapon shaped talisman - it's penetration as a weapon is that of the least penetrating effect in it so you have to waste space time and vis putting penetration on all the utility effects.
So, if I'm deducing correctly, the MoH correct version would be:
Global Penetration Mu(Cr)Vi gen
R: pers, D: con (maintained), T: group
Every effect in the item gains penetration of level - (highest level of effect in item excluding this one + 20). The effect is concentration (maintained) rather than unlimited use momentary so that effects have the penetration score even when inactive.
Very rarely worthwhile. Even more rarely worthwhile and achievable.
For the sword, you could enchant the cross only, so the blade was delivering touch range spells channeled though it, but was not nechanted itself. It is a RAW trick to overcome penetration issues in weapons and the like. No edge of the razor for the item in any case, though.
Against magical beings, just instill X eternal oblivion in your item, with unlimited uses and maximum penetration.
Even the loss of 1 might points, repeated during a battle, is sufficient to be interesting enough.