Generally speaking, you can reduce by 0.50 diopters – but quite likely you shouldn’t.
In some cases (high myopia) it can make sense to do so – see here a more detailed breakdown.
But then also reducing diopters more just means that you’re reducing your distance to blur. Seen clearly less far doesn’t really improve your vision at all – so does it make sense to make your diopter bubble smaller by half diopter?
Important note: If you want to get started improving your own eyesight, I offer a number of courses, including options for one-on-one support with me personally. Check out the courses page for what’s currently available to help your eyeballs.
Stimulus is key to improvement. Blur horizon and using active focus. Reduce only to where you get good stimulus while also seeing clearly, no more than that.
If you reduce too much you’re just skipping opportunities for good stimulus, and increase your risk of hitting plateaus.
Video version of this Q&A, on Youtube: https://www.youtube.com/watch?v=RvIpyrGYLiI
And lots more answers in the FAQ.