Temperature Converting Program

I am trying to a create a program that converts a Fahrenheit temperature to a Celsius temperature and vise versa. My form consists of three command buttons(Compute, clear, quit), two radiobuttons where you choose whether you want the entered temperatures to be converted from fahrenheit to celsius or celsius to fahrenheit, three text boxes(start value, stop value, increment), and a list box that displays the information. My problem is that the program needs to limit the conversions to 20 per computation. The program must verify that the range of values and increment will not generate more than 20 lines before any calculations are performed. I need a way to check whether more than 20 lines will be put into the list box before anything is actually put into the list box.

My current code is below.

CODE: