On substituting part sugar with Splenda

Hi,
I can't figure out how to substitute part of the sugar in a recipe for baked goods like cakes with Splenda when the recipe calls for weighing the ingredientes instead of using measuring cups.
If the rec calls for 7 oz of sugar (1 cup) and I want to use half of that in Splenda. Since Splenda is so much lighter than sugar if I weigh 3.5 oz I would end up with a lot more than half a cup of it, does it make sense?
If anybody has figured this out, I'd appreciate the help.
Thanks, ;)