a) how precise is good enough?  I.e.: just how small an error will make a measurable difference on the target?  (in something like .375 Win or .405 Win)

b) What are your preferred methods and equipment for weighing out match quality loads?

(smokeless powder)

With ball powders, +/- .05 grain, with 4227, +/- .1 gr. You might get closer than that with 4227 if your very consistant. That's about as good as it gets.

I've used a Redding powder measure with the pistol insert for 30 years for charges of 20 gr. or less. For charges up to 35 gr. I made a 1/2" diameter insert for the Redding.

If you don't mine using the B&M, it will beat any measure with 4759 and do as good or better than all the rest of the powder measures.

JMO,

Frank

My scales are only accurate to about a tenth of a grain. My old electric scale has to be constantly re-zeroed. So, tenth of a grain for me. BP, seldom weigh. Load straight out of my Redding BR. 

          Joe.

I use a RCBS 10-10 scale to accurately measure and verify powder weights to 0.1 grain (and I calibrate it for every use). With care this scale can measure to 0.1 grain consistently wereas most balance beam powder scales measure +/- 0.2 grains.

If a powder measure consitently throws weights to 0.1 grain I will use it. However for coarser powder grains the powder measures do not consistently throw the same weight. For fine powders I will use a powder measure after I verify it's consistency. For powders that do not throw well in a powder measure, I weigh each load on my RCBS 10-10 scale.

Weigh my smokeless charges to 0.x plus or minus xx

For black powder, I follow the advice of Frank Monikowski
Guys, 
I'm also going to weigh in my 2 cents. When I started shooting these cartridge rifles about 20 years ago, I weighed powder, cases, and bullets. After a few years, I ended up just weighing the bullets and the powder only to set up the measure. For at least the last 10 years I've not measured the weights of individual powder charges other then as a spot check. 
The measures I use are Lyman 55 (2), for 38 and 32 caliber. And the old Hollywood for 40 / 45 calibers. I also use a Belding and Mull for my LR muzzleloaders. I find the B+M the most accurate but the others adequate and capable of +/- 0.3 gr or slightly less on average. 

Since switching over to Swiss powder when it became available around 8 years ago, I look for a average amount of compression to determine the powder weight to set the measure to. I also use Schuetzen FG powder for reduced recoil / velocity loads. 

I load for both myself and wife. She makes wads, lubes and weighs bullets. At the risk of sounding like a braggart, between us we have 40 National Records in Open, Military, Scope, Muzzleloader and Womens catagories for BP Target rifle. And 10 National Championships between Canada and the U.S. in likewise catagories. We don't shoot silhouette. 

To me, the bullet is the most important. Weight, design, appearance and alloy. 
Powder kept around 1/2 grain total variance = good enough. Being able to break a shot down the center, call it and read wind and mirage, priceless...... 

Now this is going to upset some, but we cannot measure / weigh ourselves into the winners circle. It takes range time. I feel much of the time spent at the loading bench would be better spent at the range. I'll grant that weighing everything to a nanograin gives a sense of confidence in the ammo, but perfect ammo won't put the bullet into the center by itself. Frank Monikowski

CW:

   Trying to do everything as well as "humanly possible" is bound to be inefficient.  If you just do everything "good enough" that will be fine.   I'm guessing you are looking for what is "good enough".
    An example:    A 1 gr difference in charge weight for big BP ctgs will make a velocity difference of 5-7 fps.   I weighed a couple dozen ca. 50 gr charges from my old Hollywood measure and found the SD was around 0.15 gr.  (You can't do this experiment with your powder scale, you need a good balance.) This corresponds to ca 1 fps -- insignificant.   (You have to use some technique here, you can't just crank the handle and get that.)   If you go thru this line of reasoning, you will be able to figure out what is "good enough".
    I was given a B&M powder measure some years ago, and have never used it as it is SO inconvenient.   OTOH, I think the ritual required to operate it forces one to dump  uniform charges.  That is probably why some people swear by the B&M.   I am content merely to swear AT it.

CHRIS
RGChristensen

While we are on the subject of the effects of different powder charges affecting muzzle velocity I did a small experiment this morning to see if I could gain an accuracy advantage by upping the powder charge in my 45/90 from 77 gr of PP Wano to 80 gr.
The results were as follows.
77 gr weighed.
1st shot 1249 fps
2nd   "   1246 fps
3rd   "   1258 fps
4th   "   1259 fps
5th   "   1257 fps
Average  1253.8 fps  spread 13 fps

80 gr weighed.
1st shot 1243 fps
2nd  "    1250 fps
3rd  "    1241  fps
4th  "    1250  fps
5th  "    1241  fps
Average 1245 fps   spread  9 fps
Result, 8.8 fps slower using 3 gr more powder , Any ideas ?
Mike.

That is curious Mick.  I've heard of people loosing velocity by adding more powder, but have never experienced it myself, even with Fg in large cases with powder charges over 100gr.  How heavy is your bullet and how long is your barrel?

Regardless, absolute velocity isn't all that important.  If the accuracy is good and you have ten shot velocity SD's in the single digits then you have arrived IMHO.

Chris.