"Magic" automatic flashes, whether TTL or using a built-in sensor, are relatively recent. Before that, a handy system was developed for getting correct flash exposure manually. This is the guide number system, which is used for calculating the right mix of lens aperture, subject distance, and flash power.
The guide number itself is given in terms of distance — feet or meters. The simple formula is:
GN = distance × f-number
and of course knowing two, you can figure out the missing factor. For example, if the flash guide number is 36m, and your subject is 4.5 meters away, you would set your camera aperture to f/8 (because 36÷4.5=8).
Alternately, if you wanted a wider aperture for the same subject distance, you could decrease flash power so the guide number matches. (For f/2.8 in the above example, you'd want a GN of around 13.)
With bright ambient light, wider apertures, or if you are using a particularly long shutter speed, the natural light around you will also be a factor, and a light meter is useful in that case. But in the typical use, it's assumed that flash will provide the primary, relevant light.
How do you find the guide number? It's in your flash's specifications, and the number is slightly complicated if, like yours, the flash has a zoom reflector, which narrows the beam of light and provides an effectively higher GN. For this, you need a chart. Fortunately, the camera's manual will have exactly this chart — for your flash, it's on page 36. That chart also shows that GN decreases by the square root of two whenever you halve the power (just like the familiar sequence of f-stops for aperture).
GN is usually given at ISO 100; using a higher ISO is an easy way to increase your flash reach, and with the convenience of digital ISO, this gives you more ways to adjust your lighting even with a relatively inflexible manual flash. Since you're using film, you'll probably want to precompute the guide number table for the ISO you're using; see this answer for more.
I started this answer by referencing "magic", and it's important to note that guide numbers are not magical either. They come from a relatively simple physical property: the inverse square law. Because light propagates through space in a sphere (or a cone-shaped section of a sphere), the intensity of illumination from a given light source decreases in proportion to the square of the distance. So, one only needs to measure the power of a flash at a given distance and the rest can be calculated from there with nothing more than primary-school math.
Guide numbers are just a pre-calculated distillation of this, made exactly to provide the convenient rule of thumb you're looking for.
