Ok Backwash here goes. SG (starting gravity) should be taken before pitching yeast. Immediately after is ok but before is better. Levels to look for depend on the wash or mash. A lot of people say sugar washes should go no more than 1.080. All grain mashes probably won't get that high from my experience. Rums washes with molasses and brown sugar will be high also. Due to the heavy molasses. FG (final gravity) is taken when ferment is done and should be 1.000 or below. Again this depends on what you are making. I have sugar washes go to .992. all grains to just above 1.000. Don't forget that temperature effects the reading and should be adjusted using the formula that should have come with your hydrometer. The formula I use to calculate approx. ABV of a finished wash is "SG minus FG times 129 equals approx ABV". ie. 1.080 -.998 = .082 times 129 = 10.58% ABV. Approximately. As far as tossing the tested amount as long as everything is sanitary you don't have to toss it. But for 200 ml of wash is it that important to keep?

I just use a clean plastic measuring cup to scoop out enough to fill testing tube. I don't use any body part to touch the wash in the bucket. I will however dip my finger in the measuring cup to taste level of sweetness. To each his own I guess. % of alcohol in a wash will not necessarily tell you when the wash is done though. So you should still do a final gravity test. My hydrometer has other scales on it but I have never paid attention to them. I have an alcohol meter to test after distillation.

edit to follow when I get home from work and look at my hydrometer.

It was done successfully for a thousand years without gadgets but with them I think you get more predictable and repeatable results. Buckets from bakeries and restaurants are the way to go if you use 5 gal buckets.

Here is the instructions on my triple scale beer & wine hydrometer. "the alcohol scale indicates potential alcohol. In order to determine the alcohol content of a wine or beer you will need to take two readings: one before fermentation an one after it is complete. ie. First reading 6% minus 2nd reading 1% = 5% ABV.
Good luck and have fun.

It is just a comparable measure of sucrose level in a solution. Like Fahrenheit and Celsius. Different scales for the same thing.