How do I use an input to define decimals for other inputs?

Question

I am trying to set the number of decimals of many inputs by using a specific input.

Here is what I am doing:

window.oninput = function(event) {
  var campo = event.target.id;
  
  // DECIMALS
  if (campo == "decimalTemp") {
    var i = decimalTemp.value;
    ºC.value.toFixed = i;
    ºK.value.toFixed = i;
    ºF.value.toFixed = i;
    ºRa.value.toFixed = i;
  }
  // TEMPERATURE
  if (campo == "ºC") {
    ºK.value = (ºC.value * 1 + 273.15).toFixed(i);
    ºF.value = (ºC.value * 1.8 + 32).toFixed(i);
    ºRa.value = ((ºC.value * 1 + 273.15) * 1.8).toFixed(i);
  } else if (campo == "ºK") {
    ºC.value = (ºK.value * 1 - 273.15).toFixed(2);
    ºF.value = (ºK.value * 1.8 - 459.889).toFixed(2);
    ºRa.value = (ºK.value * 1.8).toFixed(2);
  } else if (campo == "ºF") {
    ºC.value = ((ºF.value * 1 - 32) / 1.8).toFixed(2);
    ºK.value = ((ºF.value * 1 + 459.67) / 1.8).toFixed(2);
    ºRa.value = (ºF.value * 1 + 459.67).toFixed(2);
  } else if (campo == "ºRa") {
    ºC.value = (ºRa.value / 1.8 - 273.15).toFixed(2);
    ºK.value = (ºRa.value / 1.8).toFixed(2);
    ºF.value = (ºRa.value * 1 - 459.67).toFixed(2);
  }

};

<h3>Temperature <input type="number" min="0" max="12" value="0" id="decimalTemp" name="decimal" placeholder="Decimal"> <small>Decimals<small></h3>

Celsius (ºC) <input id="ºC" type="number" min="-273.15" value="20">
<br> Kelvin (K) <input id="ºK" type="number" min="0" value="293">
<br> Farenheit (ºF) <input id="ºF" type="number" min="-459.67" value="68">
<br> Rankine (ºRa) <input id="ºRa" type="number" min="0" value="528">

In summary, I would like to know if this construction is correct:

var i = decimalTemp.value;
ºC.value.toFixed = i;

I already tried something like:

ºC.value.toFixed(i);

But didn't work. Any idea where am I wrong?