I found something priceless - Smailes foaling records. There aren`t dates of her breeding, only when she foaled though. But I did some calculating. She has had a foal on 03/12/2006 and the next foal was born on 02/21/2007 and the third one on 02/21/2008 ( how precise is that.?
) . So I figured that after Kaliostro (foal in 2006) she was bred at her foaling heat, which is ~ 9 days after foaling. That means she was bred around 03/21 and knowing her long, long heats, last day of breeding was ~ 03/25 or even after that. So in the end I figured, that her gestination length could be around 333days.
These are only guesses, but well... it`s better then nothing. Because her previous owner just did not remember how long Smaile`s pregnancies were - she has around 100 horses and 20 or more foals each year... She just can not remember what happened before 5 or more years. :)