That is probably the most effective way to do it. I like converting things to VA, doing the math, then converting back to current. It is less likely to lead one to make an error.
But you can use the shortcut of applying the turns ratio. The ratio of the number of turns in the two windings is what makes the voltage drop from 480 (primary) to 208 (secondary). The same ratio makes the current rise from primary to secondary. The opposite ratio, therefore, would give you the current in the primary, if you know the secondary.
Primary current equals secondary current times (208/480). This gives you the same answer of 54.1 amps. You will note that it is the same set of numbers, except that you don't have to multiply, then divide, by the same factor of 1.732 (the square root of three).