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.
Let's face it, these days if you're not young, you're old - Red Green