Hi Daniela,
According to my Betty Crocker cookbook, you can substitute as long as it is 65% or higher.
Hope this helps.
If you are asking about switching margarine for butter in a buttercream filling.. IMO I would not. The margarine will probably make it more yellow and besides the butter will taste better. Is this a buttercream frosting/filling, or something different?
I had to substitute margarine for butter once in a bc filling/icing.....don't recommend it. The water content in the margarine really messed it up...it looked fine when I made it, a lot more yellow, but it held together fine. As I started icing the cake, it just didn't want to smooth very well. And after I did get it iced, after a few hours, it started sweating...just oozing moisture. If you need it just for the filling, maybe it would work fine. But I wouldn't do it if I needed to ice the cake with it. 
I will use it for filling a cake at home but the texture is a lot sloppier than normal buttercream. I always use butter if it's for a customer or for coating. The butter does taste better as well
I always use hi-ratio shortening or Crisco. Everyone says they like my cakes and icing, so I figure it can't be too bad. I haven't had any trouble with icing consistency. (I usually use a meringue buttercream recipe, not the Wilton recipe.)
When you substitute margarine or shortening for butter, if you measure by volume, you can substitue measure for measure. If you weigh your ingredients, butter is usually 14 grams per Tablespoon, margarine is 12 to 14 grams per tabllespoon and shotrtening is 10 to 12 grams per tablespoon. You have read the labels and make the conversion when you substitute.
Hope this helps.
I must be the odd one out here, but I prefer Margarine to Butter. I have never liked Butter and find it to rich of a flavor!
Could also be that as a child I had to churn an old keg butter churn to make butter for hours every other week. Thats what you get for being raised on a farm with milk cows.
I also as a preschooler ate Drano crystals (never said I was a smart kid) and back then they thought butter was soothing and good for burns so I was forced to eat several pounds of it, thank goodness we know better now. But since those things I don't even like the smell of buttery foods.
I use SuperStore (up here in Alberta Canada) White Margarine and it tastes great and makes a nice True White icing when used with clear vanilla ! Never had any sweating problems or anything and I just follow the Basic Buttercream Icing recipe on the Icing sugar bag replaceing Margarine for Butter evenly. Everyone so far has liked my icing !!!
I think it is my nature to always be the odd one out !!!
RaeMarie
JennT,
I just came across this post again and I think that that's what went wrong with my cake!!!! I used margarine instead of butter for the icing and filling and it started melting!!!!!! I guess I'm better off using butter for certain things!!! Thanks for bringing that up
Daniela
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