well the achievement book is a bare bones guideline in this pack, it's only meant to show the general tech progression, the pack is what you make of it after that.
As for how much dirt should be reserved for special uses, I go with 3 pieces. One for a Pure Daisy, one for an Orechid, and one for an apple tree.
The rest of the dirt at the start I lump together and constantly use and reuse it for everything including growing more seeds, growing trees, making mana with various starter flowers (mostly endoflames), string farming, Pumpkin farming, and anything else I need a plot of dirt for.
I'm also careful to always leave some grass so when I reclaim farmland to use for something else the grass can grow back and I don't lose it.
Since your dirt is limited Agricraft is a good way to get more out of your crops in a smaller amount of space.