I'm sorry I don't have and mini shreddies to weigh for you but someone should be able to answer this. Otherwise, if you are using manual scales I would just measure 25g. Best to round down than round up if you know what I mean.
I would recommend picking some up digital scales when you can as they will come in really handy, especially if you try red or green days. Argos sell them quite cheaply if that's any help.