I am not familiar with that strobe but if you go to Youtube two links below should help.
https://www.youtube.com/watch?v=CaAghoc9yuQ
https://www.youtube.com/watch?v=borzIsfPd9M
Usually, one knob is for the strobe and the other one is for modeling lamp.
It should have an option of same power (or I don't know the actual name in English) when the brightness of the modeling lamp is associated (proportional to) with the strobe.
I would put it in that configuration so your modeling lamps will tell you visually about the lighting ratio.
It was very important that you specified that you are not going to use a light meter as that is very crucial in your getting ready.
Let me start with the basics (sorry that I assume this just to be sure - no offense intended)
1 - Light power (brightness).
Your F stop depends on the distance of the lights to the subject, the modifier (umbrella) and the total of lights. The law of light is called inverse square law.
For example if you put one light at 5 feet using an umbrella at lowest value (5), by moving the light further, twice the distance - 10feet, you loose 2 stops of light.
Without changing the config of your camera, you will need to increase the power of the light by 2 stops, which is (7).
If your light is at position (5), you take a test shot and gives you F8, if you move your light doubling the distance and without changing the power, your new F stop will be 2 less, which is F4.
(Full F stops are: 2.8; 4; 5.6; 8; 11; 16 and so on.)
If your light is already at right position (with right height and umbrella), you take a shot and it shows 7.1 and you want to shoot at F8, that means that you need to increase the power (if only one light) by 1/3rd of stop. Since the light is using a 1/10th increment, you will need to increase it from (5) to (5.3)
Hope that explained the power.
As a result, when using a strobe without light meter, the best approach is to position your lights where you want them (see below), measure the light power (F stop), put markings on the floor (or don't move the lights anymore) and when the subject shows up you just put the lights back where they were.
when the subject is really there, you take your test shot and adjust it to taste. If the subject moves towards the light, whether you move the light away of the same distance or change the F-stop (or ISO).
2 -Now the look. This is why you need modeling lamps. You move them around until you get the look you want. If they are proportional to the strobe, the final photo will look exactly the same.
But don't forget, when the subject is actually there, you might adjust the position then you need to adjust as well the power as described above.
3 - finally: two lights. The easiest way is to put them on a 2:1 ratio. Which means that if you position them equidistant from the subject, one is at power position (5) then the other one is at (6).
If (6) gives you F5.6, with the addition of the other light (at half power (one stop less)) you will be most likely around F6.3 or F7.1 and no longer F5.6 nor F8. That's because they superimpose.
With the 5D III I would use a shutter speed of 1/160th and activate as well the blinking highlight on top of the histogram (when I shoot I tend to forget to check the histogram but I chimp and the blinking highlight is there for safety.)
Hope that helps.