If a creature deals excess combat damage to a blocking creature with trample, does deathtouch reduce the amount needed to assign to the blocker?

Normally, trample requires an attacker to assign lethal damage to each blocker before it can assign any damage to the defending player (CR 702.19b). 'Lethal damage' is defined as damage equal to the blocker's toughness minus any damage already marked on it (CR 702.19b, 119.6).

However, deathtouch changes what counts as 'lethal.' CR 702.2b states that any amount of damage dealt by a source with deathtouch is sufficient to destroy a creature. CR 702.19b explicitly acknowledges this: when determining how much damage is needed to satisfy the blocker before trample overflow goes to the player, a deathtouch source only needs to assign 1 damage to that blocker — that single point is already lethal.

This interaction is explicitly supported by CR 702.19b: 'For this purpose, lethal damage is... if the source has deathtouch, any amount of damage.' So the two keyword abilities work together very powerfully.

Example: You attack with a 5/5 creature with both trample and deathtouch. Your opponent blocks with a 4/4. Normally you'd need to assign 4 damage to the blocker. But because of deathtouch, assigning just 1 damage is lethal — so you assign 1 to the blocker and 4 tramples over to the defending player.

This combination is why creatures with both trample and deathtouch are considered so efficient: they can trample nearly their full power through even a large blocker.