How to fill wall/wall gaps before taping a corner

I have put up some villaboard walls and need to tape up the wall/wall joints.

I will be tiling the bottom 2 meters, and so I am simply siliconing the 6mm gap between wall/wall joins in order to provide a bond breaker for the waterproofing membrane.

However, I will be painting the top 40cm from the ceiling.

My question is, whats the best way to fill the 6mm gap in this area before taping the joint? I find that the pre-mixed "wet area" base coat I am using tends to shrink too much and doesn't really seem suited to fill the 6mm gap.

Should I perhaps fill using cornice cement? Silicone?

Had a look at Rod's "How2Plaster" site. The video on taping corners seems to imply to use base coat, but that doesn;t seem to be working well for me.


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