I am adding a new bath and using pex to plumb the supply lines. It is only 10 feet from the hot water tank. All the existing supply lines are 1/2 inch copper right from the meter which is 35 feet at the other end of the house. Question is all the manifolds I see have a 3/4 inch inlet. Will I lose pressure if I use a 1/2 to 3/4 adapter to tie into the manifold which will have 1/2 inch outlets. Thanks

You will loose pressure and volume if another 1/2 inch feed line off the manifold is opened ( at the same time another 1/2 line is running )... Thats why the 3/4 inch feed to the manifold.