Here is a good article on int/exh pressure ratios. Under the sub heading "Turbo cam basics"
http://www.hotrod.com/how-to/engine/ctrp-1106-turbo-camshaft-guide/
Ideally, you want less exhaust pressure than intake pressure. A lot of it depends on the turbo manifold design and size of the turbine wheel and housing and exh pipe size exiting the turbo.