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If you have the GPS Status (&Toolbox) app (https://play.google.com/store/apps/details?id=com.eclipsim.gpsstatus2&hl=en) you can use the "Pitch" to determine the angle. U90 would be vertical so for 15 degrees just subtract. 90 - 15 = U75.
How To Make The Low Cost Inventory Storage System
Kid's Pallet Bed