Do they both go down same percentage?

[Randy Holmes-Farley Verified Community Expert]

If phosphates go down 50% due to water change will nitrates also go down the same amount or even close to that amount? I have no HR nitrate reagent so I'm doing math trying to figure out about how much nitrates i have.

No. Water changes remove a higher fraction of nitrate than phosphate from tank water because a lot of phosphate is bound to rock and sand, and if you do a 100% water change, some will come back off the rock and sand to replace it. That is not true of nitrate.