Let me tell you something - Tradução em português

"I'll tell you something/one thing/another thing also let me tell you something/one thing/another thing used to emphasize what you are saying" --Longman Dicionary

Exemplos do emprego da expressão:

I'll tell you one thing - you'll never get me to vote for him.
Let me tell you something - if I catch you kids smoking again, you'll be grounded for a month at least.

Literalmente significa "Deixe-me contar alguma coisa", mas acho inadequado. Alguém tem uma sugestão melhor?