For a start I bought the small Alpkit tarp, I could not find a way to keep it fully over me in a freestanding position, I just didn't have the knack to erect it properly. When I did use it I felt that I was just 'too close' to nature, and my imagination ran riot, mice and the like. I did like however the ability to open my eyes and 'see' the world around me. For my own comfort I realised that I am a person that needs to be enclosed! I can see however that with the right Bivi, the tarp could be a really useful addition if I can learn its permutations properly.
In order for me to seriously consider a Bivi, it needs to be big enough to take me, my sleeping quilt and my Neo Air blow up mattress. I think I have worked out, that in the way I do camping, I would need to take my larger ground sheet to go under the Bivi and have some sheet still available for me to o stand on in my bare feet, changing, etc. Or have two ground sheets, one under the Bivi and one to stand and/or sit on.
I would also need to keep my boots dry, safe and in a secure place. I never leave them outside my tent, just in case they 'walk' in the night!
Last but not least, when in the Bivi where would I put my rucksack? Again, I would need it to be dry, safe and in a secure place?