Anyway, here's a quick stat in light of last night's final:
Sampras 18 final appearances in which he won 14 times and lost 4 times; therefore, he won 78% of his total Slam finals.
Federer 24 finals; won 17 times and lost 7 times; therefore, he won 71% of his total finals.
Nadal 19 finals, won 13 times and lost 6 times; therefore, 68% of his total finals.
Nadal's % will change of course in the future with each loss and win as he's the most likely player to face both going forward as Sampras is a retired player and Federer is on his way down, so don't feel his percentages are likely to change all that much in the future.