If you use a lower cut-off grade you will include more "less mineralized material" in your total tonnage calculation.

PRB most likely used a cut-off grade of 30% Cr2O3 because above this grade the economical processing of Chromite is more likely.

If BT has nearly double the tonnage between 25% and 30% cut-off, that means that a lot of material is very low-grade and more difficult to process economically.

That´s why I wouldn´t go with ressource estimates quoting tonnage below 30% cut-off.

And yes, you should compare apples to apples, which means 36 Mt for BT and 8.4 Mt for BC.

FANTOMAS