Some time I'd like to further research the topic of the metallic gene being dominant, and get others' thoughts from their experiences. But, I think you may get either all metallic (hikari) or half metallic and half non-metallic, depending on the female's genetics. It depends upon the female, not because she's female, but because she is metallic.
The presentation below indicates that the metallic trait (versus non-metallic) is controlled by one gene pair, where the metallic gene is dominant, and the non-metallic gene is recessive.
http://www.koi-bito.com/forum/attach...i-genetics.pdf
So IF it is really that simple, that would mean that your showa males each have two recessive genes and your platinum female has either one or two dominant genes. If she has two, then all of the offspring will be metallic. If she has one dominant and one recessive gene, then half the offspring will be metallic and half will be non-metallic.
I once bred a metallic female with 4 males (2 metallic, 2 non-metallic). Once I got the numbers reduced to 15-20, I recall they were all metallic. (I do not recall if there were any non-metallics among the hundreds.) In retrospect, I think that particular female must have two dominant genes.
Please keep us posted on how your percentages come out.