What's the preferred method to set consistency font size and line height for a website using em?

What is the best way to set cross-browser typography (font size and line height) for an entire site using a em

fixed width {Width:970px}

, centralized website?

I usually get a design from a client with multiple font sizes and line heights in different places in the design. for some reason i am still usingem

without getting problems with nested elements and font size inconsistencies in IE and others.

after installing the way of easy management and update and calculation ems

I want to set easily manageable font sizes, and I want to set the line height manually (because it can be different for different places in the design.

And for what things should we determine the line height or for which not?

How to set font size and line height for a consistent result. and if i use em

to select the font then should i also use the bottom edge h1, p, li, etc. in em

?

    HTML {}

    BODY {}

    P {}

    a {}

    ul li a {}

    ul li ul li a {}

    p img {float:left}

td,th { }