By my math, MSB shipped 2.7M tons through Q3. They shipped 0.98M tons in Q4. This means:
0.29M tons @ 5% base royalty
0.69M tons @ 5.5% base royalty
The base royalty paid was $2.86M, which implies a price of $54.46/ton.
The bonus royalty was $270k, and that amounts to 0.5% of $54.46*0.98M tons. 0.5% is the lowest bonus rate, for actual prices no more than $2 above the adjusted threshold price.