....
If you are sailing downwind....my experience in monos shows a daily
run of anywhere from 130-180 nautical miles a day. ...

Those seem like reasonable numbers to me, but be aware that the
variances might be quite high. Nothing is for sure with a sail boat.
Even on the same routes at similar times of year I've found that my
passage times can vary by a factor of two, and I'm not alone in this.
There was a boat sailing near me on my last passage up from New Zealand
who had arranged a complete crew change in Tahiti on the assumption
that he could make 100 miles a day no mater what. Sadly he ended up
hove to for a week and found himself a week late and in the Cooks... I
typically plan passages in stages. At a preliminary stage I look at
the pilot charts and make a rough guess at how long a passage will take
and then double that number to make allowances for waiting on weather
and such. This can take place years ahead of time. Starting a few
weeks before the intended passage I start looking at weather charts
daily to get a feel for the macro weather and readjust my expected time
based on the weather I think I'm going to be sailing in. Finally, when
I check-out I look one more time at the best wx models I can get and
make a final estimate of my passage time and then tell everyone that I
plan to take twice as long as I suspect I really will. YMMV.

-- Tom.