The tax will be based on the full value of the salaries is all. If they prorated the tax the prorated salaries would add up and max out in the same way. It's just easier to do the math if you work with the original salaries (i.e. the Phillies have $2.8 million and adding two vets to the roster would cost $3 million).
But the tax is also not determined until the end of the year, so if they replace some of the vets with younger players or are deadline sellers, they won't hit it. Conversely, they don't really have room to add a player of any note if they're contending, unless they could get someone to take Herrera in the deal.
Norris had an opt-out, he probably wasn't heading for the 40 anyway (in which case he wouldn't count against the tax until called up) with some of the other guys ahead of him. Same with Drew Storen at the start of camp. Even without the tax they don't have room for more than two relievers (if that) on the 40 and Storen wasn't one of the top two or three.