This is how you find the right dimensions for your whirlpool lid.
When measuring length and width, make sure to give us the measurements in centimeters and always round up to the nearest centimeter. When measuring, a little too much is better than a little too little. One centimeter too much in the length doesn't make much of a difference, the cover still fits, but one centimeter too little and the hot tub cover no longer fits at all.
Classic hot tub
Measure at the widest point of the acrylic edge as shown by the arrows
Whirlpool with railing ring
Measure at the widest point on the railing as shown by the arrows
Embedded in the ground
Measure at the widest point on the floor as shown by the arrows
Here you can see the difference in folding
The dashed line shows the folding of the lid
Measure the radius correctly
The most important thing is that you do NOT choose the radius too large. If you measure the radius too large, it will NOT cover the radius corner. If you measure the radius too small, that's not a problem. The lid may protrude a little, but that's hardly noticeable. If in doubt, always measure a little smaller than specifying a radius that is too large.
The radius is the measurement from the point of the extended corner to the start of the curve.
If you don't have a square at hand, take a wooden slat, cardboard, etc. and measure the distance with a tape measure or ruler