What the spreadsheet tells us is that you will have better than 2.5% accuracy if you:

• round to the nearest hundred sats/$ when bitcoin is between 4000 and 2100 sats/$ ($25k to about $48k)
• round to the nearest fifty sats/$ when bitcoin is between 2100 and 1000 sats/$ (about $50k to $100k)

Another way to put it is "above 2100 sats/$, round to nearest 100, below 2100 sats/$ round to nearest 50."

This works up to $100k/BTC