The Alert Percent value is the percentage of weight difference BELOW the baseline weight that will trigger an alert notation on the chart.
For example, let's say the current baseline for a pet is 100 (whether it is grams, ounces, kilograms or pounds) and today, the pet weighs 95, then:
95 - 100 = -5 (95 minus 100 equals a negative 5)
-5 / 100 = -5% = Maximum allowable weight drop.
A weight of 94.5 will be a -5.5% difference from baseline and WILL trigger an alert.
Our default is 5 percent. You can change this value to suit your needs. If you are weighing a sick animal daily, then you may want a tighter (lower or smaller) percentage. For example, a 3% drop is a more conservative number to monitor for. You would be looking out for a smaller drop to trigger some additional action, such as medication or a vet visit.
According to Wikipedia, the average adult female person in the US weighs 164 pounds. A 5% drop in weight would be 8.2 pounds. Unless you are super tall and 'big-boned', most people would consider going from 164 to 156 to be within a healthy weight range and not cause for alarm.
If you have an "average" adult female guinea pig of say, 800 grams, then a 5% drop in body weight would be 40 grams (or 1.4 ounces). If you have a bigger than average adult female guinea pig, say 1200 grams, then a 5% drop would be 60 grams (or 2.1 ounces) -- which is a 50% increase in an acceptable weight drop!
But what if you have a baby guinea pig? A 60-gram baby can certainly not tolerate the same actual weight loss. This is why the percentage is important. And a 3-pound adult male would typically need to lose more than a few ounces to send you to the vet.
It's just too hard to say whether weight gain is due to normal growth or pregnancy. And typically, weight gain is a slower process and while important, is usually not as immediately life-threatening or indicative of illness that may require urgent care.
For weight gain monitoring, some of the more subjective body assessments come into play, such as BCS or Body Condition Scoring.