Strange usage of というもの


**I need help understanding というもの in the sentence
貴方一人を御し得ないようでは私の器も知れるというもの.**

I am reading Fate/Stay Night: Unlimited Blade Works and one of the servants
"Caster", is going to use her blade to take control of another servant. Since
he is a traitor she is not sure if it would be the best move and she is
hesitating on whether or not to use the blade. He told her that he could
betray her, but since the conditions are in her favour, as long as Caster has
the upper hand he will not do so.

Her response:

> 「令呪とは、裏切り者を罰する為にある、でしょう？  
>  ……いいわ。 **貴方一人を御し得ないようでは私の器も知れるというもの。**  
>  もとより貴方の“宝具”には興味があった事だし、思惑にはまってあげましょう」

My understanding:

>   * Verb-ますstem + 得ない = Cannot Verb.
>   * 貴方一人を御し得ない =I cannot control only you
>   * ようでは = If [with negative result]
>

My translation:

> Command seals are there in order to punish traitors, right?  
>  Ok. If I can't control you alone, you will know my (tool, device which in
> this context is a) blade. From the beginning I had an interest in your
> "Noble Phantasm", I will go on with your intention."

Fan Translation:

> "The Command Spell is there to punish the betrayer, right?  
>  …All right. I shall prove to the world that I am great enough to take you
> in.  
>  I am interested in your Noble Phantasm, so I will follow your plan."

**How does というもの add on to the nuance of the sentence?**




It's some ancient accent invented (or adapted?) by the author, you will never
hear any real person say that. "というもの" here is close to "だもの" or "ですもの" or
"ということだけ", which emphasizes that the person is very confident about what he /
she said.

But "器" in this sentence doesn't mean "device", in the common phrase "器が知れる"
it means one's power / caliber / capacity to control and lead others.




That …というもの（だ） means that it's natural for everyone to consider it that way.
I'd translate it to "Without being able to control you alone, that **should**
be the limit of my capability".

