first time using n sensor passive

[turbokid]

hi. i have an old n sensor with x 20 controler. i know its out of date and pron to give bother, but im not intending in using it that much.. today was my first day to try it, just playing really. it was on winter wheat, and the settings i put in were growth stage 25/tillering, target rate 99kg/n, min rate 50kg/n, max rate 150kg/n and bio mass cut off was 20%. it was the second split of fert and most of the field was light/pale and the sensor was reading low crop density in alot of areas. but where the crop thickened up it seem to work fine reducing the rate and where it read low density in just applied the target rate. just wondering if i should only use it on the next/last split to get the benifit if any. thanks