I've a problem figuring out how to calculate stops for Interfit EXD 400. In the product manual it says that the strobe has 5 stop range in 1/10 stop increments, but the numbers displayed on the strobe itself aren't stops, it's in watt power value starts with 13 and ends at 400. So how to calculate the stops from these numbers?
By "Watt power" we basically mean radiant flux, and by "stops" we mean luminous flux. These two have similar definitions, but they are different.
In normal life, it is true that if you halve the "Watt power" and that light is reflected from or refracted through linear (normal, not non-linear optical) materials, then your camera will observe half the luminous flux - therefore if you halve the "Watt power", you create the same effect as if you stopped down your camera 1 stops (by changing aperture or shutter speed).
If you use a digital camera, your life is easy. Since you are using a studio strobe, your shutter is fixed, you are smart so your ISO is fixed ;-), and also your aperture is arbitrarily fixed. So make a test shot, see your histogram. It maybe shows you are off 2.5 stops. So just multiply the wattage power by 2^2.5. You will have a great setup with a few trial-errors, and will quickly learn to be intuitive with the controls.
An stop is simply a doubling of power. If you are at 200 watts and increase to 400, that's another stop. Lighting is drastically dependent on distance and angle to subject, so while the relative contribution of a light will be impacted by increases in stop, the absolute contribution of the light is based on not just the output of the light, but a large number of other factors. This is why flash power is normally measured in either a guide number or watts of total power and why proper metering is necessary to setup lights precisely.
For a flash, fractional power is usually based on the total power of the flash. For example, on my Canon 600EX flash, 1/1 is a full power flash, 1/2 would be one stop below max, 1/4 would be two, 1/8 would be 3, 1/16 would be 4, 1/32 would be 5, down to 1/128. For your flash, 400 watts is full power, 200 is 1 stop reduced, 100 is 2 stops reduced, 50 is 3 stops reduced, 25 is 4 stops reduced, 13 is 5 stops reduced (well technically 12.5, but they probably rounded).