Battery AH Confusion

You are correct in saying that you used 57.5AH.

If your battery is drained at the end of the 150 minutes, you would have a battery capable of 57.5 AH at a C2.5 rate. It's capacity at a C20 rate would be much higher. (The published AH rating for most batteries are at a C20 rate, although some MFG use the C100 rate so that they claim a higher rating.)

My guess is that your battery has a much higher AH rating. Cycling a battery at a C2.5 rate would result in a very short life of the battery. If you know the make and model of your battery, the specs are easily found. Dimensions can often be used when a model is not known.