You're in the same boat I am right now. Here is the way I understand it, and if anyone wants to jump in and correct me, that'd be great.
First off, use this calculator to input the skill you already have on the left side and then the skills you want to have when your character is finished on the right side:
http://sro.mmosite.com/x/
This will give you a total number of SP that you will need to get from where you are at right now to when you are done with your character at level 70.
Now look at this chart:
http://stud3.tuwien.ac.at/~e0125557/sro/spgain.html
This chart shows how much SP you will gain per level using each level gap that you might decide to use throughout the game. Gran a calculator and add up the column starting at where you are (in this case level 21) and add up all the SP between level 21 and level 70 using the level gap that you would like to use the rest of the game.
Now subtract this amount that you just added up from the total amount generated by the XP calculator from the first site and you will know how much SP you need to farm before prcoeeding with your normal level gap to level 70.
As far as I can tell that's it. Proceed to tell me I'm screwed up if I am, this is just what I've deciphered from reading other people's posts, and if I'm wrong I'd like to know now before I spend a long time farming up more SP than I have to.