Summary

A six inch protractor. Will be accurate if your printer is accurate.
To measure angles, align the inside straight edge with the bottom side of the angle, with vertex at center line of the rule..
Angles are marked every 10 degrees with 45 degrees marked also.
bottom edge serves as a straight edge, and 6 inch rule.

I made this as my first project in FreeCAD...

Print Settings

Printer Brand:

Ultimaker

                                            <p class="detail-setting printer"><strong>Printer: </strong>                     <div><p><a rel="nofollow" href="https://ultimaker.com/" target="_blank">Ultimaker 2</a></p></div></p>                                                                                <p class="detail-setting rafts"><strong>Rafts: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting supports"><strong>Supports: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting resolution"><strong>Resolution: </strong>                     <div><p>.15 layers</p></div></p>                                                                                <p class="detail-setting infill"><strong>Infill: </strong>                     <div><p>18% infill</p></div></p>                                                        <br>                        <p class="detail-setting notes"><strong>Notes: </strong>                    </p><div><p>takes about 1.5 hours to print.</p></div>

Protractor