What was assumed is the frequency separation always been 6.25Khz.The calculator appears to assume one base frequency and a step size of 12.5 kHz. The sites below do not follow those assumptions and give wrong answers from the calculator.
Not the base frequency, it's calculated using a known LCN+frequency and 6.25Khz. Now if a different frequency separation is used then it will be wrong.
DSDPlus does not show the frequency separation value (5, 6.25, 10, 12.5, 15, 20, 25, 30, reserved) that I'm aware of. While easy enough to add to calculator, it add another unknown (x9) in to the mix.