A guide number (GN) is the power of the flash to illuminate a subject correctly at ISO 100 at a specific distance using a specific f-number.

Normally this distance is 10 ft for easier calculations, so the GN is

GN = 10 x f/number

The thing here is that we need to know if you have a correct exposure so the first approach would be to use an incident light meter.

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But if you do not have one, you can use the histogram on your camera.

1) Prepare a target, a white wall for example. This part is tricky because a different color will render the histogram in a different way and this white wall can potentially render a histogram graph clipped on the right side. Point the camera at the center of the illuminated zone.

2) Put the flash at 10 ft from the wall. You can put your camera closer so you do not have to worry about zooming in and out, remember that the GN does not care about your camera, it is about the flash.

3) Take a shoot using F6 as an aperture, because the flash declares a GN of 60 and see your histogram.

The GN is a number at full power at 200mm, but I would change the power of the flash to make this histogram to be in right on the middle, so depending on the wall, probably you need to lower the power at 1/2 or 1/4 for example.

This shoot is the "calibration" this combination of settings is your reference.

4) So now start zooming your flash without changing the other settings. I would not do the full range of the flash, I would personally just do 100mm, 50 and 20.

Take a shoot and adjust the aperture so your histogram is again in the middle. If you used now, for example, f/5 the GN is now 50; if your histogram is ok at f/4 the GN at those settings is now 40.

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If you have a gray card, you, in theory, do not need to lower the power of the flash to have the histogram in the middle.

Using an incident light meter will give you more exact results.