How to Measure 1 Grain of Powder Accurately?

Nice.
I would have just used a vernier for depth measurement.

There are good suggestion on the above posts. if you must weigh and be accurate , you will need a Chemical Balance or Torsion Balance scale and a set of Gram weight that will weigh in 1 gram Fractions. Meaning 10th of a gram, 100th of a gram, and so on.
If your not aware of what a Chemical or Torsion is , there are plenty on Ebay to view .

I use the Lyman 55 for very small charges.I load the .25 ACP and the .32 ACP.The drop weight doesn't vary.I use a balance beam scale to set the measure.

For my 32 ACP loads for my Pardini target gun I use a Dillon with a Unitek measure. I throw 1.12 grains of TiteWad and 1.5 of Tite Group and have no problems. I also write on a sheet of paper where these loads are on the micrometer scale and it is very easy to get back to the same spot.

GONRA has loaded lottsa .25 ACP using 1.3 grains and 1.5 grains WW 321 Powder.
Start out with RCBS Chargemaster.
Check / touchup on beam balance using Powder Dribbler.

Make yer own funnel arrangement to dump into the tiny .25 ACP case.
(For me that was the greatest problem. Took some time to get stuff together.
Tiny "ring stands" and funnels are needed here!)

Use a "Hornady Custom Grade New Dimension Bullet Seater Die 25 ACP"
to properly seat tiny 35 and 50 grain bullets.

Any time you find a good deal on new .25 ACP Brass , JUMP ON IT!
Finding yer fired cases in the usual "range brass" pyle is almost impossible.....

Have Phun!