Tack Strap Ruler
To check your extension numbers, use the ruler on the tack strap and measure from the top of the collar, where your mast sits, to the lowest position on your extension (just above the pulleys). This will tell you if the extension marks are accurate.
The amount of extension does not affect how the calibration gauge works. The calibration gauge is independent of the base extension amount. For example, you could have your extension set 10 cm too much and this would not change how the gauge works. You would still just downhaul until the gauge lines up with the bottom of the mast and the net result would be your sail would be 10 cm higher off the deck, but the actual downhaul tension on the sail would be set correctly.
If on the other hand, if you set your extension 10cm too short, you would bottom out on the pulley well before the gauge reached the bottom of the mast and this would tell you that you need more base extension.