You are concerned with two things: (1.) making certain that the amp you buy will not overheat when it supplies your speakers with the current they will draw when producing the sound power you require, and (2.) making certain that the amp you buy will not clip when it supplies your speakers with the voltage they require to produce the sound power that you require.
As for the first concern ...
I = (Power/Z)^(1/2)
When the current drawn by your speakers at the power you require is the same as the current implied by the amplifier specification (at the amplifier's rated power), this will be true:
power_required / my_speaker_impedance = rated_power / impedance_used_in_the_spec
rearranged:
power_required / rated_power = my_speaker_impedance / impedance_used_in_the_spec
For example, if the ratio on the right side of this equation is 1/2 (i.e., the impedance used in the amplifier spec is twice greater than the impedance of your speakers), then since the ratio on the left side will also be 1/2, this means that in this case the rated power of the amplifier will need to be twice as great as the power you require.
As for the second concern ...
V = (Z x Power)^(1/2)
When the voltage required by your speakers to produce the sound power you require is the same as the voltage implied by the amplifier specification (at the amplifier's rated power), this will be true:
power_required x my_speaker_impedance = rated_power x impedance_used_in_the_spec
rearranged:
power_required / rated_power = impedance_used_in_the_spec / my_speaker_impedance
For example, if the impedance of your speaker is twice greater than the impedance used in the amplifier spec, the rated power of the amplifier will need to be twice greater than the power you require.
Except for when the nominal impedance of your speaker is the same as the impedance used in the amplifier spec, exactly one of the two potential concerns will apply. You don't need to be concerned with which concern is the one that implies. Divide the smaller impedance into the larger impedance. Multiply this by the power you require, to obtain the implied amplifier power rating.
This is all very theoretical. If the calculator you used took into account the sensitivity of your speakers and the impedance of your speakers, and if it also factored in the "crest factor" (to compensate for the difference between the nominal power rating and the instantaneous power), it stands a chance of being useful. But I would still go more by my experience. If you've used an amplifier rated for 100 Watt per channel and you never had any issue with overheating or with distortion due to voltage clipping (or with not being able to turn it up higher than the volume control will permit), then you likely do not need more power than that, especially if the speaker sensitivity and impedance have not changed, but probably even if the speaker sensitivity and impedance have changed. Many audiophiles assume that their system will sound better if they replace the amplifier with a more powerful amplifier. Of course there will be specific cases where this will occur, but it is not true in any general sense that using a more powerful amplifier will improve the sound of your system. If the old amp sounded like trash because it was clipping, a new amp may fix that, or if the old amp got hot and shut down, a new amp may fix that, or if the old amp wasn't loud enough even when turned up all the way, a new amp may fix that. It is unlikely that the new amp will sound any different than the old amp sounded when the old amp wasn't turned up to the point where it sounded like trash because it was clipping.