I would charge the same, no matter what the type wood or what sizes. The bigger the dimensions, the harder to handle.
Try to shoot for $65.00 per hour, and see what it comes out to at a bd/ft rate.
Any time you are not putting the blade into the wood, you are losing time-money. That's why the reference to hourly rate. You may find you have to raise the rate by bd/ft, to equal the $65.00.
Softwood-hardwood, it doesn't matter, IF you have the mill tuned correctly and running efficiently. You will spend as much time handling logs-lumber-slabs, as you actually do sawing.
Do you have any experience with a band mill ??