Estimating Inductance
Ferrites vary in \$\mu_r \$ from maybe 10 to more than 10,000, so you're likely to do better measuring the inductance and calculating \$\mu_r \$ from that. 

Here is an example inductance meter project (there are plenty more out there, no endorsement is implied).  

In fact most of the ones on the net (especially those using an LM311) are unabashed copies (or copies of copies) of the original AADE design- he was kind enough to publish the schematic and algorithm used, making it easy to copy. If you can afford it (~$100), it's better to get the original. 

You can use your formula to estimate the inductance of a new coil wound on that core, so all is not for naught.