This expression is used in an affirmative context, not negative: "I can't tell you exactly what it looks like, but I can give you a rough idea." -or- "I don't know which car is his, but I have a rough idea." Словосочетание don't have a rough idea не скажит никто, но don't even have a rough idea можно: "My Spanish isn't good enough to read this, I don't even have a rough idea of what it means." ...даже не имею смутного представления.