hallo, well there is no real answer for that, but its a good idea I think to study your favourite manga and comics that use them and see how they’re implemented, and when. There are screentones that are for just filling in where colours would be, but there are also tones for emotional scenes or moments, or to strike a mood. A key point is doing less screentones than more on a page will be better, and try putting them down meaningfully, that way since you will be more limited anyways. Hope this helps a bit (´・ω・｀)