I would do it one gallon at a time, you don’t want to over correct and end up too low. I don’t know of any calculator to help you, but I would just do it conservatively. That said, I don’t think you need to do this over several days, I would just do it over several hours. Also, I would personally shoot for 1.026.
Edit: as I mentioned there is no need to do this over several days, it is not going to hurt anything by doing it over several hours. If it were a much larger difference, then stretching it out more might make sense. In the wild, corals experience fairly large swings in temp and salinity throughout the day. A lot of the corals we keep come from tropical locales that get substantial rain, and if they’re from inshore shallow reefs they also deal with the runoff from the shore, so on a given day, the salinity can change by several ppm. People always preach stability, and stability is great, but I also feel like people sometimes go too far in trying to keep things stable and end up stunting their corals’ and fishes’ ability to handle changes. You see people who keep every parameter completely stable and then if some minor change happens, the corals react negatively, but then there are people who allow some fluctuations and when they experience a change, their corals don’t react whatsoever. I think this is topic that should be explored more thoroughly, but I think the hobby-wife move towards more stable and sterile tanks is overall detrimental. *I want to be clear that I’m not suggesting you keep water parameters that are way out of range, but a little bit of controlled instability is beneficial. Also, sorry for the rant.