i think if you try to measure accurately you'll be in for a bad surprise. after laying folded for a long time, it may be ripply and out of square. but with all those bias edges i would be afraid to tackle pressing it. i'd do what moonpi said and also hand pat it as flat as possible. slip a large piece of paper under the open triangle and follow the outlines the missing shapes. add your seam allowances. and pray.
you may find it's easier to do by hand. or, if hand work gives you the creeps, then at least hand baste. hand stitching gives the best control and lets you ease where you need to.