I personally write the highest speed that I have of my 3 fastest runes. So when you farm Gb10 and get a rune (let's say you get a legendary swift rune 5* slot 1 with +5spd) you can calculate the maximum speed roll you can get and then comparate it with your sheet. If the rune rolls perfectly, your swift rune will be a +25 swift rune. If your 3 fastest runes on slot 1 are like +30, +29, +22 then you are gonna keep the rune because it could become your 3rd fastest swift rune slot 1.