How much water to use when cooking pasta in an Instant Pot
You're correct, the water ratios may be slightly different between different types of noodles. In general, you're covering the noodles so that they will actually have a chance to soak up the water though, as with a pressure cooker, you don't have a chance to stir the noodles and ensure equal saturation.

When making pasta in the instant pot, you'll also need to keep in mind the layering effect of your ingredients. Tomato based sauces are prone to burns, so you want to keep them on the top of the pot. Put your noodles on the bottom, and if you feel that it's taking an excessive amount of water to cover them, you can leave the last 1/2 inch or so exposed, so long as you cover it with your tomato sauce, or some other liquid.