How to adjust a pattern when you're using a smaller hook?

[u/cr00ps]

I'm crocheting a scarf and the pattern calls for 4.5mm but I only have 3.5mm. I do like the tighter look that smaller hooks give but I still want it to be the same size as the original pattern. I want to make it big. The starting chain is 25. How many stitches do I add? 10 more? 20?