I tried to find a fix for the problem where the division of the total cost by the number of payers results in slightly less than the total cost being paid e.g. $10 total cost divided by 3 payers results in $3.33 per person when it should really be $3.34 (a few extra cents tip, rather than a slightly lower tip - especially when 0% tip is selected).

I can identify when the problem happens by doing the calculations with Decimal instead of Double but, sadly, I couldn't come up with a nice, elegant way to adjust the per person payment amount and be able to inform the user that a little extra needed to be paid in total.

If anyone managed to come up with a nice solution I'd love to see it.

I think I tried messing with pennies. But then I realized I HATE pennies!

Instead, I added a toggle named "Round Up?" next to the per person cost. It's on by default. This is my version of a nice solution.

 @State private var roundUp = true // Attach this to the toggle. Prefer to round up!
 var checkValue: Double {
       guard let value = Double(checkAmount) else { return 0 }  // read the string. Total check value
       return value > 0 ? value : 0.0 // Positive values only.
}
var basicPayment: Double { (checkValue + tip) / Double(totalSeats) } // standard formula
var totalPerPerson: Double { roundUp ? basicPayment.rounded(.up) : basicPayment } // Round up!

That being said, you should probably take a close look at this forum message:

Message: Don't use Doubles for Currency