I have just received this response from Fermentis to my question about making a starter with dry yeast.
"Once you have rehydrated the dry yeast (see rehydration tab in the "tips and tricks" section on www.brewwithfermentis.com), the yeast is in the same condition as any other yeast in liquid. If you do not rehydrate before you start agitating, you can end up decreasing viability due to the cell membranes of the yeast cells being in a fragile state. If you rehydrate the yeast appropriately, you can propagate the same as you would any other yeast."
The Brewers Friend yeast starter calculator does have a Dry Yeast tab.