Do everything in your power to have upper cabinets as tall as possible. At all times try to avoid soffits or open areas above your cabinets. They are unnecessary and you will benefit from all the extra cabinet space. Good luck with your project!

36+18+42=8' so you need 10" of crown to fill it yo the ceiling. This could mean a 8" riser above the cabinets with crown at the ceiling.

My kitchen ceilings are 8'. My cabinets do not go all the way to the ceiling. Most have 9" of space between the moulding and ceiling and some 3". I love how it looks and it gives the illusion of more ceiling height.

I don't know how you'd feel about the look, but you might be able to save some money if you were open to using a flat trim board plus some quarter round rather than crown for your top trim. I recently remodeled a kitchen in a 1930s house. The ceilings are very wonky and unlevel. My cabinets run up to them, and crown molding would have made it impossible to make the trim flush against the ceiling and also level. So instead, my cabinet maker just used a trim board and some quarter round. I actually like the simplicity of the look and it was much less expensive than crown would have been because the trim board is way cheaper and the cutting and mitre work is faster.

Here's what it looks like (sorry for the not great lighting).

I love the flat trim! What size trim and length of cabinets did you go with?