If working on the desk top, raise your chair until you have approximately 90 degrees at your elbows when typing and/or mousing. You may need to add a footrest for ideal leg support.
OR
If using an adjustable tray, raise/lower the tray until you have approximately 90 degress at your elbows when typing and/or mousing. No changes to chair height should be required.
Your mouse must be at the same height as your keyboard.