1 L is probably close to what you need...I think that's my diff's capacity.
If it was me, I'd use the RL, close it up, and order another bottle to add to it...or just wait until you get the extra you think you need...which you might...but not much more at all.
I'm sure it's not horrible what you're planning, but the safe move would be to use the same fluid. If you beg at dealer, you know you can get the OE diff fluid...whatever that is.
I'm so depressed. No one wants to come see me. :(
Mangofy your E46!