How much thrust does your boat actually need?
Most sizing charts ask for your boat's length and nothing else. Both Sleipner and VETUS say in their own documentation that length alone is not a valid way to size a thruster — and published charts disagree with each other by roughly two times for the same boat.
So this one works from the thing that actually matters: how much wind your topsides catch, and how hard the thruster has to work to win that argument. It gives you a range, and it shows you the reasoning.
What this is doing, so you can argue with it
The wind load on your topsides is standard fluid drag — half the air density, times wind speed squared, times a drag coefficient, times the area the wind sees. Nothing proprietary about it.
The thruster doesn't have to overcome that whole force. It has to win a leverage argument about the point your boat pivots around. Because the thruster sits near the bow and the pivot sits near the middle, the thruster's lever arm is long — which is why the number it needs to produce is a fraction of the total wind load.
The reason we give you a range rather than a number is that the gap between where the wind pushes and where your boat pivots varies from boat to boat, and it's precisely that gap which makes the published charts disagree. Anyone quoting you a single confident figure from your length alone is guessing with more confidence than the physics supports.
Size up, not down. The most common regret in owner reports is a thruster that can't move the bow in a real breeze. As one owner put it: a thruster that can't hold you against 20–30 knots isn't worth having at all.