Is it sufficient for one who has not calculated how much zakat is due on them to pay an amount exceeding what is owed, with the intention that the surplus be considered a charity?

If a man pays a sum of money with the intention of zakat, an amount not less than what is legally obligatory, then his obligation is fulfilled. What exceeds the obligatory zakat is considered a sadaqa (charity) for which he will be rewarded, as Sheikh Ibn 'Uthaymeen mentioned.