Not a waste of time in my experience. As for the calculation, as the secondary progression is the yearly progression, the tertiary is the monthly progression.
While the secondary progression equals one day in the ephemeris with one year of your life, the tertiary progression equals one day in the ephemeris with one lunar month.
http://www.astrologicalpredictions.ca/news/tertiary.htm
I find that especially close to exact aspects to tertiary or natal angles and/ or the tertiary Moon are often present at significant events.
One very significant event took place in the week between 9th and 13th june last year for me.
my tp Moon 6°48 Gemini
my tp Sun 7°44 Gemini
my natal DESC 7°06 Gemini
my sp Sun 6°08 Aquarius
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another decidedly more painful one happened on 15th july 2013.
Tp Moon 8°09 Capricorn
sp DESC 9°45 Cancer
sp Moon 10°01 Cancer
n Pluto 9°05 Libra
Tr Pluto 9°57 Capricorn
Tr ASC 9°03 Cancer
Tr Moon 10°09 Libra
This was the day my grandmother my passed away.
Natally she had
Pluto 8°27 Cancer
Mars9°03 Cancer
Sun 9°59 Cancer
Of course these are just personal anecdotes, but at least for myself I have followed my tertiary progressions, and they were very reliable in the frame of the secondary progressions/ natal
The secondary progressions will still herald the big bells, the tertiary progressions are more like a finetuning tool to see when it might happen.
EDIT:
Unfortunately you can`t rely on astro.com for this. They will just take the first day of your tertiary progressions and give you the position of the Moon on that day. HOwever the Moon moves about 13° (approximately, it varies) on any given day. And since one day in th ephemeris equals one month of your life, the Moon would be on a different degree, let`s say 2 weeks into the month you are checking.