I am a fan of making the primary OCPD protection as large as permitted, even if it makes my wires bigger.
I second that emotion.
A 75kva 480d-208y/120 has been one of my most problematic issues that I have have as a former sales and applications engineer.
All to often those in the field are backed into a corner when the panel that they intend to feed the transformer with is limited to 100a frame breakers max. The spike of inrush current is the issue that can cause nuisance tripping of the PRI. breaker at the point which the transformer is energized. The minimum inrush of a standard efficiency 150degC, 75kva transformer is about 10x the PRI. FFLA which is 900a. As such a 100a breaker which has a 1000a max mag. trip would be marginal. With 115 and 80degC, k factor transformers the inrush may increase anywhere from 20-50%.
As petersonra stated the higher the OCPD the better.
Remember that the secondary OCPD protects the transformer from being overloaded, the PRI. OCPD protects the upstream distribution system from a transformer failure due to an internal fault. The pri. OCPD really has little to do with protecting the transformer at all. As such try to use as much of that 250% as you can. The only other consideration is a question of protecting the cable between the PRI. OCPD and the transformer.