New Didn't weigh jar

Weigh the jar with the starter inside.  Write it down.

Remove the starter to another container preferably one with smooth sides and big opening, like a small bowl.  Use a rubber or silicone scraper so that all the starter lands in the bowl and jar is very cleanly scraped out.  

Now weigh the jar and write down the weight.  (You don't have to wash it. You can do that later sometime when the jar is completely empty and clean and make a more accurate reading.  For now it is good enough.)

Keep the jar on the scales and set the scales to "0."   Pour and scrape the starter back into the jar and don't forget to clean off the scraper as well getting all the starter back into the jar.   How much does the starter now weigh?

You can double check your figures by adding the jar weight and the starter weight together. You should come out very close to the original weight of the starter filled jar.

Have fun and let us know if you run into any problems.  

Mini

Most scales will read below zero, too, so sometimes I do this: put the starter jar on the scales. Press the Zero/Tare button. Scoop out spoonfuls of starter until it reads say -200 g (if I need 200 g of starter).