Yes that is what actually is missing in the CKL, what if the N2 is above 15 %. Cause the CKL wants you to start the engines on APU only when neither start is succesful and N2 is below 15 % .The light up time in air is 30 sec, so should we go for another attemp of recycling the start levers, if 30 sec is up and N2 is still ( by any chance ) above 15 % .(specially if you dont have APU)