Pretty simple : in the column "I have" you enter the number of runes you have in each level. In the column "I want" you enter the number of runes you want in each level for you it's 3 of level 5 and 3 of level 7. Then in the column on the left "ratio" you give the ratio you want to apply for each level (personnally I put -5, the best imo but I can be wrong).
At the end at the bottop of the page you have the total ammount of crystal chips + gold dust you ll need to get your runes.
Hope I have been clear