2.

Q: 
How well should computers be synchronized for 
synchronization to work as accurately as logical time?

A: 
In general, so well that the sending of a message happens before receiving it.
More precise:

rtt is the round-trip time, t1 is the time at site S1 and t2 is the 
time at S2.

S1 sends a message to S2. With logical time the sending of a message
happens before receiving it. So S2 should never think it has received a 
message before S1 thinks it has sent it.

I.e. 
t1 < t2 + 1/2rtt =>
t1 - t2 < 1/2rtt

So the difference between the clocks of two computers must be smaller than
half of the round-trip time. 

Lets see what happens if this is not satisfied.

1) t1 = 100, t2 = 79, and rtt = 40. S1 sends a message that travels 20 time 
units until it reaches S2. So S1 sends at time 100 and S2 receives at time 99.

2) t1 = 100, t2 = 79, and rtt = 40. S2 sends a message that travels 20 time
units until it reaches S1. So S2 sends at time 79 and S2 receives at time 120.