Well, either you research your spells in the old-fashioned way. This involves, basically, comparing the spell to others like it on the same spell list. I think there are books that offer "benchmark spells" which are considered the upper limit of this spell level's power (magic missile is the benchmark spell for first level).
If you want to build your spells using something like seeds and make this the main method of spellcasting in your campaign (or the only one), get Elements of Magic. The PDF is less than 10 bucks and allows you a completely flexible spell system. Not only do you get magic points (we can as well go ahead and call it "mana") instead of prepared spells, you even get to assemble the spells yourself instead of getting prebuilt spells, so this is not even more flexible than D&D magic, but more flexible than most other magic systems out there.
You get several "spell lists" per level. A spell list consists of an action type (charm, compel, infuse, evoce, abjure, transform, summon, and so on), and a target type. This can either be an alignment (the classical ones from D&D), a creature type (again, the classics like undead, dragon, humanoid, and so on) or an element (which there are much more than the 4 classical ones. We have the classicals like fire or water, life, death, paraelements, positive elements, negative elements, and some unifying stuff like nature or time).
Each action type lists what can be done with the list, how much spell points each enhancement costs, and so on.
