Hi. Glad you like the app.
You're not the first to ask for this. It shouldn't be too hard to add. I can see a number of ways of doing it...
1) Have a checkbox on the "Adjust portion" screen. Something like "Suggest the new item in future [x]". This should probably default to whatever setting was used last time.
2) Have a checkbox on the item add/edit screen. Something like "Suggest this item in future [x]". This should probably default to checked, as it would be present for every item you add, not just items created by adjusting a portion.
3) Have a checkbox tucked away in the settings screen that has the same effect as (1). I.e. only applies to items generated via "Adjust portion", but is more set-and-forget.
Personally, I don't feel a need for this feature -- I tend to have two or three portion sizes hanging around and a combination of those is usually close enough -- So I'm not sure which of the above would be best. I'm inclined towards (1) as it is un-intrusive but is in front of you when needed. What do you think?