Important thing is to choose the correct trinkets. I am running thru HC modes with like 7-8k spirit (+flask, 300 food where needed) but Darkmoon card (equals to +-1200 flat spirit) and Scroll of Revered Ancestors (like 900 spirit) = roughly 11k spirit.
Another really important thing is usage of CDs - I am innervating at like 90% of mana, popping regen trinket @ pull basically. I am specced to Incarnation and using it usually as a mana saving CD - so use it as soon as you can (but dont waste it ) Prepot intellect (especially if you start with HOTW dps) and make sure you use Focus potion.
So for me 10-12k spirit (including trinkets) feels OK on progress fights - I like to give myself an option to spam reju/regrowths if needed without disabling myself.