the way i do it is use the calculator
http://sro.mmosite.com/x/ and the sp gap chart
http://stud3.tuwien.ac.at/~e0125557/sro/spgain.html to figure out how much ill need vs how much i get at each level. then i just see how big of a gap ill need till a certain level and go with that. for example, im lvl 42 right now. i need 85k sp for what i want. so i have to let my gap increase till 45, then go 9 lvl gap until 50. theni can close it to 0 and have all that i want.