**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?**