How have you structured your tech purchasing process to make sure TCO calculations are accurate?

We have a standardized project and operations TCO and ROI model developed by the Finance team. No funds are released until Finance signs off
Then at 1 year 3 year and 5 year mark the TCO and ROI are checked to confirm it provided benefits

To my knowledge nobody has been fired for missing the targets but perhaps some were encouraged to seek employment elsewhere

Yes to greater extent 

We have very careful planning processes for tech purchases and consider very carefully the anticipated TCO. We also keep some wiggle room, such as redundancy and fail -over plans  in the process to prevent surprises. So far our TCO calculations have been spot on. Measure twice - cut once.

I worked at a  K-12 private school that places a great deal of importance to the use of technology in learning and teaching.
We have what we call an IT Replacement Cycle, where based on  the lifespan of  equipment, we plan the replacement and plan to set aside the funds accordingly. Most of our devices fall in the 4 or 5 year replacement cycle, but others need to be replaced sooner and a few last more than 5 years, so our planning cannot be the same for  each year, however, knowing in advance what we need to replace and what investment is needed, helps us have devices that meet our standards at all times.

We have not structured it in such a way to make sure those calculations are accurate. We do our best to interpret the data that we have on these purchases, but I'm not confident that they are always accurate.