Hi Charles, Steve,

Using a variable cap is essential to get the desired accuracy. What I do in my digital clock with date, I get a bag of 1000 caps of 33 pF. Then I do the software tuning and mark the caps bag for this software.

One bag is good enough to make 500 clock units. By the time one bag exhausts I get another bag and tune it. So for every unit you don't have to put in an extra dollar. Do the software tuning and buy the caps of good tolerance in bulk.

For accuracy I prefer Sand clock :))

Simon