hmmm, its a bit complex because just because its from a double sized loaf doesnt mean you just have one slice, usually healthy extra b' come to 6 syns so therefore if you can work out how many syns is in a slice then work it upto 6 and then you have your healthy b choice. i always buy 400g loaves just to stop the hassle of this happening
I just buy a wholemeal loaf of any size, medium sliced and use 2 slices. The thing I love so much about SW is it's simplicity and the lack of weighing and measuring. I did weigh HE stuff when I stared it but now I estimate it all and it works that way for me