You can "open" R1 for 6/18dB filters by setting R1 to a very large value, e.g. 1000000.
You can "open" C5 for 6/18dB filters by setting C5 to a very small value, e.g. .0000001. Using 0 (Zero) will result in an error.
2/25/21 Added more data points for smooth curves at low frequency.