JSFiddle - React, Tailwind, and code Playground

HTML

<form name="form1" >
<table border=0 cellpadding=2 cellspacing=2>
<tr>
<td align=center>Сбитых ВЦ/НЦ:<input style="border:1;" type="text" name="c" size=7>
                 Смертей/Битв:<input style="border:1;" type="text" name="d" size=7></td>
<td align=center><input type="button" value="Считаем" 
onClick="calc1(this.form)"> Ваш КПД:<input style="border:1;" type="text" name="total1" 
size=7>%</td>
<td align=center></td>
</tr>

</table>
</form>

JavaScript

var inp = document.getElementsByTagName('input');

document.onkeypress=function calc1(form) {
a = inp[0].value;
b = inp[1].value;

c = a-b;
k = c/a;
d = k*100;

inp[3].value = d;
}