In general, modifiers affect the thing nearest to them. For instance, “of the medium” modifies “usefulness”, and “for graphic artists” modifies “medium”, or it might be a second modifier for “usefulness”; I don’t see a useful distinction there.
The problem is “in a letter”. The nearest thing to modify is “artists”, but that makes no sense. So you try “medium” and then “usefulness”, which also don’t make much sense. Finally, you try “forecast”, which does make sense and is therefore probably what was meant. (If it didn’t, you’d try “he”, which gives the same meaning.)
Here are some other arrangements:
- He also forecast in a letter the usefulness of the medium for graphic artists.
- He also in a letter forecast the usefulness of the medium for graphic artists.
- He in a letter also forecast the usefulness of the medium for graphic artists.
- In a letter, he also forecast the usefulness of the medium for graphic artists.
Note that the closer “in a letter” is to “forecasts”, the less ambiguity there is about what it modifies.
Note that if you put it in these two places, it radically changes the meaning of the sentence because the nearness rule’s effect on the other modifiers:
- He also forecast the usefulness in a letter of the medium for graphic artists.
- He also forecast the usefulness of the medium in a letter for graphic artists.