15 miles at 5,000 and 250 you are high, unless very light and/or strong headwind. That is how you would be with a FLx3 rule.
Anyway, I always prefer to calculate parting from my present altitude, speed and configuration. Distance to go is not clear every time and a don't need gates or specific sets of conditions.
I must say that most of the times, when FMGS FPLN is what I actually fly, my rule takes me with the donut. Only if conditions are special (weight and wind, basically) I deviate from that.