Mitty wrote:
How (calculator, flight computer, mentally, etc.) and how precisely,
during an approach, do you calculate your time to the MAP based on your
ground speed crossing the FAF? Finally, to you, what is an acceptable
"bit?"

A perfectly reasonable question. Lacking any better instrumentation
(DME, or GPS), here's what I do:

1) Estimate the surface wind based on the best information you've got
(which usually means ATIS or AWOS).

2) Add some random fudge factor to account for the fact that the winds
at 500 - 2000 AGL (where you're going to be flying the approach) will
tend to be a bit stronger than on the surface.

3) If it's not a direct head or tail wind, take a WAG at the
head/tailwind component.

4) Add this to the indicated airspeed you plan on flying the approach at
(at the speeds and altitudes you usually use for approaches, IAS is
close enough to TAS that you shouldn't worry about the difference).

5) You now have a reasonable estimate of your groundspeed. If you're
trying to refine this estimate to anything closer than the nearest 10
kts, you're fooling yourself.

6) Now, look at the FAF-to-MAP timing table and do a rough interpolation
between the listed entries.

If you spend more than about 30 seconds on the whole process, you're
working too hard. Without a reliable way to measure GS, the best you
can do is a reasonable guess, so don't knock yourself out trying to do
anything fancier.

With a handheld GPS, you're be insane not to take advantage of the
information it gives you. If the MAP itself is not in the database, set
it navigating to the ARP, or the FAF, or the last stepdown fix, or
whatever makes the most sense for that approach. Start your watch to be
legal, but use your GPS to be safe.