An RBI can be used just like an RMI. Forget there are any numbers on it and forget its got a bottom half. Just mentally transpose the NDB needle on to the DI and you'll be close enough. Remember that if you need to make a correction then it needs to be big enough to make the needle move in the direction you want. Inbound correction is fly same direction as pointer but slightly more otherwise it won't move (push the point), and when outbound then fly away from the tail (tug the tail) again by a big enough amount to make it move. Don't worry about the maths, just transpose the needle. Simple. I taught IMC in a knackered old C150 using the Cranfield NDB and it worked well enough. I've also done raw data NDB approached in a 737-800 admittedly with an autopilot and RMI but it still works !