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Using the plastic practice horizon bubble sextant, you do have to make a correction value. There are Two ways of doing it, one with the natural ocean horizon the other is with a known position. Make sure your sextant is first adjusted correctly and do a few Solar IC adjustment or calculation. Keep that figure / value written down. After everything is adjusted correctly put the practice horizon on. Bring the suns Lower Limb to the line. Now take about 30 sites from a known position. Then adjust the error with a bubble correction. Remember the correction has that I.C. Error already added to the correction. This is why you need to know the error because your BUBBLE CORRECTION will change, it will change with the I.C. Error. Plus it gets you into the practice of knowing your sextant intimately. Ok always, Keep doing this until you can draw our a slope line and visually see the errors. Then average out the errors. Once you have an average, apply them to your sites. See if your sites begin to line up with the slope line. When it’s does you got your bubble correction plus your IC error. Now write down your bubble correction SUBTRACT out the I.C. And you have your base error of the unit itself. Each practice horizon has its own error value. Mine was about 13.72 as an average.