What is the best method to set cross-browser consistence typography (font-size and line height) for whole site using em for Fixed width {Width:970px}, centered website?

I usually get design from client with multiple font size and line heights at various places in design. for some good reason i still use em

without getting nested element problem and font-size inconsistencies in IE and others.

then after setting how to manage and update easily ,and how to calculate ems

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

And for which things we should define line-height or for which not?

How to set font-size and line-height to get consistent result. and if i'm using em for font-sizing then should i also use bottom-margin of h1, p, li etc in em?

    HTML {}

    BODY {}

    P {}

    a {}

    ul li a {}

    ul li ul li a {}

    p img {float:left}

td,th { }