<script>
var mb = document.querySelector("input[name=medidab]");
mb.addEventListener("keyup", calcImc, false);
function calcImc(){
   var md_val = parseFloat(mb.value);
   medidab = (md_val/50*100).toFixed(1);
   if(!isNaN(medidab)){
      document.querySelector("input[name=circunferenciabraco]").value = medidab;
   }
}
</script>

<div>
   <input class="campo-form-c" type="text" name="medidab" placeholder="Medida do Braço" maxlength="5">
</div>
<div>
   <input class="campo-form-c" type="text" name="circunferenciabraco" placeholder="Circunferência do Braço" maxlength="5"></br>
</div>