How accurate do you find the hydration recommendation is?

I did 30 min slow walk during my lunch break and 20 min run this morning before work just over 2.3km and my recommendation for today is to drink 3.5l of water. According to garmin I have sweated nearly 300ml of during my lunch time slow walk. I always feel that it overtime by a lot on some activities. I doubt that I sweated any more than I would being sat in the office.

Is it just ne or are everyone else recommendations all over the place?