The stronger the contraction, the more exercise you do and the faster the muscle will increase in strength, you will need to feel the stimulation quite strongly to be sure to exercise the muscle properly. In principle, you should turn up the strength as high as you can stand it. You should at least feel the muscle start to pull upwards, this is a good level at which to start your first session. However, when you first start exercising any muscle strongly, you may get aches afterwards.
When you are starting, it’s best to set the strength quite high, but limit the length of the session to five or ten minutes. If you don’t get aches the next day, you can gradually increase the strength and duration. As the muscle strengthens you will find that you can increase strength and duration. If you set the strength much too high, you may experience aching the following days.
People vary in sensitivity to electrostimulation - there are lots of variables that can affect what you feel - however most people won’t feel much below about 20.0, and some will use the maximum 99.5. Before your first use, try moistening the probe, holding it in your hand, and adjusting the strength to see what it feels like - remembering that your hand is actually much more sensitive than your pelvic floor.