This worked but not sure if it was the best solution.. Was there an easier way I didn't think of?? I created a computed property to add the tip and check amount:

var tipPlusTotal: Double {
        let checkTotal = Double(checkAmount) ?? 0
        let tipAmount = Double(tipPercentages[tipPercentage])

        let tipTotal = checkTotal / 100 * tipAmount
        let total = checkTotal + tipTotal

        return total
    }

and passed it in here:

Section(header: Text("Check Total")) {
                    Text("\(tipPlusTotal, specifier: "%.2f" )")
                }

I can't say whether it's the best way, but it's very similar to mine! Not sure if that's good or bad! ;-)