Shallow water soloing is not a lot of fun.
Armchair landings from any real height will hurt, badly. In addition to water slap and bruising, its fairly easy to damage your neck/back too. Try to avoid landing on your ears too as you can pop your ear drums.
An armchair landing is a compromise between pain, how deep you want to go and how much time you have in the air to sort yourself out.
As flat as possible will keep you shallow.
http://www.bbc.co.uk/news/world-europe-12773427