@ JieJenn, I have attached the file that I'm working (in the first post after I've edit). Forgive me if you see the code in mess, since I'm not a programmer.

@ MaczaQ, How would you do this then, if you were me. Basically, trying to accomplish these few things.
1. Instantaneously change the amount when user input.
2. User able to input with decimals (i.e. 25.2)
3. Show '%' sign after user have finish input the data in the textbox.