Having read the PDF file from Brian, that top formula seems wildly familiar.
With that said, is an inrush current sinusoidal? (as in a pure sine wave?).
We wind our own transformers at work (PoCo) and one part of my Linemans apprenticeship was 6 months in the Transformer shop, which I only go into these days if it's too cold in the Faultsman's office!.
I must ask our head Transformer guy at work how they calculate the inrush current, because we get a Post-It note tacked to the side of the tranny stating what size fusing to use, for both run and in-rush current.
Bear in mind Chris, we use totally different voltages here in New Zealand, but the formula should still hold true.