～るまでだ・～たまでだ: the difference
～るまでだN1
Meaning
Just, Merely, Simply, Only
Past papers
Example
「問題が起きても、解決するまでだよ。」
'If a problem arises, I will just solve it.'
～たまでだN1
Meaning
Just, Merely, Simply, Only
Past papers
Example
なんで怒っているの？私の意見を言ったまでのことだ。
Why are you angry? I merely stated my opinion.
How to tell them apart
「Vるまでだ」 (dictionary form) looks ahead: a resolve for the worst case.
「Vたまでだ」 (past form) is about something already done: 'I only did what I should, nothing more'.
「だけだ」 just says 'only' and lacks the 'last resort' feel.