Svelte - Binding a checkbox to change an input value

Using svelte, I want to set the default value of an input based on whether a checkbox is checked or not. The input is used in a drug dosage calculation. The calculation takes an value of weight in kg (k) x the input value.

I also need to have the drug calculation results change when this input value is changed by the checkbox action or when a user changes the input value manually, which doesn't happen currently.

I have been able to implement the input value change when the checkbox is checked but not clear on how to get the calculation to recalculate when the checkbox is checked or the input value is changed manually.

I need some help in integrating the correct input value into my calculation.

Checkbox:

let yes = false;
<input type=checkbox bind:checked={yes} >   

Input:

<input value={yes? item.Fdosevalue : item.dosevalue} step={item.dosestep} 
min={yes ? item.Fdosemin : item.dosemin} max={yes ? item.Fdosemax : item.dosemax} >

Calculation:

Not sure how to integrate the checkbox change in this calculation.

<span bind:this={k}> {( (k * item.dosevalue)).toFixed(1)} {item.appendvol} </span>

Here is a REPL which will hopefully make it a bit clearer