I actually use it to optimize for how much to contribute because when the stock falls you end up hitting the convoluted IRS limit, which degrades the effective return on the total amount of contributions.
To effectively figure out the 25k limit you need a history of prior contributions to know how much limit is left in each tax/calendar year.
But you are correct, the 25k limit is based on all of the purchases made in the calendar year. And the $25k carryover complicates things as well.
Because it's always worth contributing the max to ESPP no matter what happens, then selling as soon as possible, if you have that. None of the tax optimization stuff is worth it. It's just an option to get 15% more money in exchange for getting paid once every six months.
My company limits us to a maximum of 15% of your total compensation in addition to IRS limits. I set it to 15% and let them sort out the IRS math.