Personally, I sp farm in interims. If I'm not farming I usually have a 0-1 difference.
When I see that i need to farm I have a special method

:
First I decide at what level I'll stop, for instance, I'm level 23 and I need a total of about 1.5k sp at 25 for berserker arrow, second book imbue, etc.. so obviously I will need to farm.. a lot..
