While there may be room for more sophistication it seemed to me from your original post that the Arduino ADC was giving a number of about 420 when there is nothing on the scale and 900 when it has it's maximum load.
That suggests that you are getting the full range of what the scale is capable of.
Your original question was about weighing 100 coins (500 grams?) and it seems to me the scale can't do that.
You said in your original post "After that it goes off the scale." but that isn't true if the maximum reading is "900 (me pressing down on it)".
Changing the amplification won't change this unless of course you are trying to work beyond the capabilities of your amplifier.
If you have a suitable meter try measuring the voltage at the input to the amplifier with nothing on the scale and with a heavy (maximum) load on it.
...R
PS, if I was you I would ask the moderator to move this Thread to the Project Guidance section of the forum. ...R