I personally don't do secondaries at all, I think it's medium-low risk and low reward, and more work and cleaning than you really need. Take the gravity reading over a few days as long as it's stable and the reading makes sense based on your recipe then it's time to package. Unless you are dry hopping, in which case it is time for that.
As for what to expect relative to OG, you want to calculate the ADF (apparent degree of fermentation) also known as apparent attenuation. To calculate this, subtract the FG in points from the OG in points, then divide by the OG in points to get a percentage.
For example, a beer that starts at 1.050 and finishes at 1.015 has an ADF of (50-15)/50=.70 or 70%. Whether that makes sense or not depends on the yeast, mashing temp, and ingredients in the beer. If you post your recipe, OG, and FG we can help you figure it out.