Tl;dr, you can't.
There isn't actually such a thing as "power level", but we can mostly pretend that there is. However, it's impossible to calculate such a thing accurately, and it's also not the only thing that matters - think about consistency. A wildly unpredictable swingy deck might win very quickly on good starting hands but fail completely other times, or you might have a very powerful deck that's completely shut down by certain "silver bullets" that it has no way of dealing with (for example, many reanimator decks, or my own Acererak deck which is probably higher-end casual but dies to anti-storm type stuff).
Still, in an attempt to assess your deck...
- It doesn't really seem to have a clear focus. I'm not sure what the plan is, honestly (there's some counters stuff, but it's not universal). Your commander says, let's play creatures, you can't stop me, they have trample, I will trample you with uncounterable big creatures. But then you have a lot of random non-creature spells that don't seem to support this strategy. It kinda looks like you just threw a lot of good cards you like together, which is fine, but not going to be powerful.
- It has a relatively high average CMC but only 30(!) lands and not much ramp. A lot of the time I think you will be stuck with expensive cards in hand and no way to cast them.
- It has a few powerful cards (mostly expensive "bombs") but also some rather weak cards.
- Your removal suite is kinda weird. I don't think you have any wipes, you have like a fight spell and the mana-intensive Turn to Slag, then you have a couple of enchantments which are actually OK but don't seem super on-theme. Return to Nature is fine here.
- You have a couple of random instants and counterspells that seem kinda odd. Why Lofty Denial? This isn't a fliers deck. Why Inscription of Insight? What does that do for you? Usually with a deck like this you probably are playing battlecruiser, dropping big threats on your turn, so you won't have mana to hold up for counterspells and if you do you risk not playing anything at all.
- You have some card draw, but not real repeatable engines or anything that's that good.
As a result, I think this is probably weighing in below many precons; the card quality is similar on average, maybe (precons have a lot of chaff, but usually some good stuff too, or stuff that's OK except for being a bit mana-intensive), but precons tend to have a clearer theme and also actually better (if not brilliant) removal. I don't think a number is useful, honestly, but if you really want one, it's probably like 3 or something.
I would think of PL 6-7 decks as not necessarily having combo wins or the very best cards but probably having healthy ramp, removal and draw suites and possibly tutors or powerful cards like Docksides or whatever but more likely just playing them for value. This doesn't really have any of that so is definitely lower. If the manabase meant you could actually play stuff, maybe this could be a 4-5 because you do have a couple of powerful cards there, but right now I think it will struggle even to do, like, anything, which implies it's a level or two lower still.
(Pulling some random hands, I really struggle to have useful stuff to play turns 3-4, it's mostly land, pass, land, pass, and I sometimes have to work even just to get some draft chaff type creature out)
Now obviously if you're on a budget or whatever maybe even swapping
Shock for
Lightning Bolt etc is not worth it, so fine, I'm not judging or anything, this is just an assessment. You may not want to make this deck any more powerful - if you are playing against similar decks, no worries.
However, I would suggest upgrading the manabase, which can be done cheaply, because I think this will just be un-fun otherwise. Honestly, cut some of the chaff and just replace it with lands. You can probably find a list of "best ramp" and just go down it in order until you find things you have or can afford (or just proxy) and slot in a couple of pieces.
Sword of the Animist can be great,
Farhaven Elf is totally serviceable, but you could surely put in
Sol Ring,
Arcane Signet,
Wild Growth,
Overgrowth, maybe some other two-mana ramp options. Also there are plenty of budget dual lands which help with colour fixing a bit - I suggest filter lands like
Fire-Lit Thicket, maybe bouncelands like
Gruul Turf.
EDIT: Huh, turns out those filter lands are less budget than I thought. Maybe people are catching on? Still, other options do exist.
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