I grilled two racks of spares and a bit of chicken yesterday on the Santa Maria...the ribs look similar to yers. This was in yer drum? Mine I did over a hickory fire. Mine were not burnt either.
As for temperature I set exhaust wide open and intake on 2. Kind of a look and feel. About the only thing I use gauge for is that temp is going up or down. Think I mentioned above I went just a tad long. Not much but some. I think your 3 hours 250° would be really close