Looks like you have an issue in that the value is being evaluated only when loading. Add some code like the following to watch for changes. I do see that you mention that there are other fields that will also be included. Add the field number in the $('#Field95').change so that if you were also adding Field94 it would look like $('#Field95, #Field94').change(... and then add the code to construct the variable V8820 before assigning it to make the coding easier and in one place.

$(document).ready(function(){
	$('#Field95').change(function(){
		// added Field 95 10/28/19
		if( $('#Field95').val() != 0 ) {
			var X =$('#Field95').val();
			V8820 = "ECEAP (8820)    %" + X + Nline;
		}

		$('#Field96').val(V8820);
		// Field 96 is the multiline text box.
		/// there are many other lines and variables added to the value
	});
});