A transformer of that size was final tested when it was fabricated. Efficiency at different load levels is part of these tests. There is a hard copy of a test report somewhere that would be a great help to you. Of course, if the unit is 20 years old, it can be long gone or difficult to locate. Do not fine tune it, but there are losses without load (iron) and loss of load (copper). Both are important and together are total losses. The efficiencies of the transformer are high, 98 to 99% commonly.