i want to get the GST percentage of 10% in the total bill amount, how would i do that with javascript

I am creating a online estimator with vanilla js, now at the end i want to calculate after the total cost i want to add 10% on top that. How would I add the 10% in the total bill amount with JS?

Now how would I get the total bill amount which includes GST as well.

Please help me with this, I’ll be really thankful.

Thanks and Regards in Advance

Here is my code

 var product_total = document.getElementById('product_total');
        var call_cost = document.getElementById('call_cost');
        var gst = document.getElementById('tax') / 100;

        
        // Decrease Number //
        const decreaseNumber = (incdec,itemprice) => {
            var itemval = document.getElementById(incdec);
            var itemprice = document.getElementById(itemprice);

            console.log(itemval.value);

            if(itemval.value <= 0){
                itemval.value = 0;
                alert('Negative Quantity Not Allowed. Please Select Atleast One Service');
            }
            else{
                itemval.value = parseInt(itemval.value) - 1;
                itemval.style.background = '#fff';
                itemval.style.color = '#000';
                itemprice.innerHTML = parseInt(itemprice.innerHTML) - 70;
                product_total.innerHTML = parseInt(product_total.innerHTML) - 70;
                net_amount.innerHTML = parseInt(product_total.innerHTML) + parseInt(call_cost.innerHTML);
                document.getElementById("net_amount_total").value=  parseInt(product_total.innerHTML) + parseInt(call_cost.innerHTML);
//              document.getElementById("service_name").value
            }
        }
        
        //Increase Number
        const increaseNumber = (incdec,itemprice) => {
            var itemval = document.getElementById(incdec);
            var itemprice = document.getElementById(itemprice);

            if(itemval.value >= 10){
                itemval.value = 10;
                alert('Maximum Number of Services Upto 10 Allowed');
                itemval.style.background = 'red';
                itemval.style.color = '#fff';    
            }
            else{
                itemval.value = parseInt(itemval.value) + 1;
                itemprice.innerHTML = parseInt(itemprice.innerHTML) + 70;
                product_total.innerHTML = parseInt(product_total.innerHTML) + 70;
                net_amount.innerHTML = parseInt(product_total.innerHTML) + parseInt(gst.innerHTML) + parseInt(call_cost.innerHTML);
                // net_amount.innerHTML = parseInt(gross_amount.innerHTML) / 10%;
                document.getElementById("net_amount_total").value=  parseInt(product_total.innerHTML) + parseInt(call_cost.innerHTML);
                 
            }
        }

  <div class="summery-list">
    <div class="icon">
      <img class="img-fluid" src="https://wefreelancer.in/love/ntcc/wp-content/themes/ntcc/images/bathroom-color-icon.png" alt="">
    </div>
    <div class="title">
       <h4>GST</h4>
    </div>
    <div class="price">
       <h4><span id="tax">10%</span></h4>
    </div>
</div>