Hi there all you clever developers!

I need your advice - I'm a designer, well 60% a print designer and I'm moving into web and I would love your opinions of what you need included in a spec sheet from me the designer. Previously I've handed a developer a psd and expected it to come back identical and we've ended up back and forth 'tweaking'. I'd also like to use a spec sheet to get clearer quotes from a freelance developer on the cost for the yet undesigned site.
Any advice you guys have is appreciated!