It depends on the attenuation of the yeast you use.
Each extract will have a specific gravity based on the fermentables. The don't ferment 100%, so you cannot calculate ABV based on x amount of DME/LME.
Oh I don't know. I was just thinking it would be nice to know how much extract I could use when making a brew. I'll probably be doing hef most of the summer. I think the most for that style is around 6%. So I was thinking that it would be cool to know how much malt to order to achieve that result. Is beersmiths calculations generally pretty accurate?