I can't get my calculations to work depending on whether the check box is check or not.

First of all, congrats for naming your field the way you did.  It will render the code much simpler.  Instead of hardcoding your arrays the way you did, save time by populating your arrays with JS.  The push() method of the array object will append an element at the end of you array by updating the original array:

var aCheck = []

var aKm = []

for (var x=1;x<=20;x++){

    aCheck.push("Check."+x)

    aKm.push("KMsRow"+x)

}

notice that I used "a" as a prefix for arrays.  It is easier to remember that this variable contains an array.  Use prefixes like f for fields, v for values, n for numbers and s for strings.  Trust me, as a begginer, this will help you identify bugs much quicker.

Unless you absolutly need these arrays somewhere else, you do not have to declare them since you created a loop afterwards that loops 20 times.  So either you create arrays and loop throught THAT, OR, you loop 20 times and do want you need in the loop.

to loop inside ALL the elements of an array, use the "in" command like so:

for (x in aCheck){

   console.println(aCheck)  //will print all elements of the array to the console

}

to evaluate if a checkbox is checked, do not evaluate it's value as you did, use the isBoxChecked() method instead.  it returns a boolean.

Do you need this to be a function?  You created a function that as no argument ***function myKM("Nothing here")*** yet you are trying to pass two arguments to it ***myKM(km, check)***.  That will not work.

So unless you need the arrays, I would not use them and go strait with the loop.  Copy this in the custom calculate tab of you "Total" field

var sum = 0                                                                            //Will store the sum

for (var i = 0; i < 20; i++){

var cCheck = this.getField("Check."+x).isBoxChecked(0)     //stores true or false

var vKm = this.getField("KMsRow"+x).value                         //stores the value of the current row

if (cCheck) sum += vKm                                                        //If true (checked), adds value to the sum

}

event.value = sum                                                                  //assign a value equal to the sum to current field