The easy way I found to make a starter for something like a Baltic porter is to make a standard porter like an American or English type first. After that you have plenty of yeast cake and don't need a starter. Just put the Baltic into the same fermenter and let it rock. Now you have two beers a nice porter and a Baltic and no guess work over how much starter you need. You should have no problem going up on OG from a standard Porter to a high gravity porter like a Baltic and it will be ready for Christmas...