The actual forward speed can be measured via a radar gun or some other measurement tool... just for info 1 mile/hr = 17.6 inches/sec

The question about the tip chord, that is a good one....

Kindly refer the attached diagram, the center drawing represent most of the production prop profile, to actually simulate it, the sectional chord can be measured individually in a fashion shown on the bottom picture. However in my spreadsheet, like i said before, i modeled it as a straight taper as shown in top picture. The root chord is the largest chord while the tip chord is the last available parallel chord a long the airflow direction.

If you choose to measure each individual sectional chord, you may modify the cells F5 to Y5 to accommodate your dimensions. In fact for a propeller designer this will be a very good tool to simulate the best possible profile of a prop where the thrust produced at max while keeping the total negative torque down.

Let us know your findings... Thanks.