There are some mistakes in the tutorial:

1. Crystals are many more stable than 10%,
typically 50ppm = 0.005%
10% can you only get with a simple RC oscillator.

2. The given timer routine can not count precise.
See at : http://www.specs.de/~danni/ap.../index.htm
for exact counting.

3. To adjust a trimmed capacitor you need a very high precision
frequency meter. Otherwise its a painful job, since you
not know, how every turn change the frequency.
So its better to check the deviation in seconds over about 14 days
and correct this in software.
M y clock, I have build in this way, have about 30 seconds deviation
over the year.