Each pipette contains 50 micro grams of ivermectin (in a carrier oil).
The safe range for a guinea pigs is between 200 - 400 micro grams per kilo of body weight.
Your girl weighs just over 0.5 kilos, so a safe dose for her is between 100 and 200 micro grams of ivermectin.
This is calculated to be within a very safe range, whilst also being most effective.
You have given her 2 pipettes, which is only 100 micro grams of ivermectin, which is slightly under the minimum dose for her weight.
If you give her 3 pipettes (the recommended dose) she will still only have had 150 micro grams of ivermectin, which is still 25% less than the maximum possible dose for her size.
It is very important not to overdose and you are right to be careful, but by under dosing you run the risk of not curing the problem completely. This can also cause resistance to these treatments, making the problems harder to treat in the long run.