Which engine are you talking about WG?
For the RR we always used min N3 of 25% (RR recomended).
On the PW JT-9 20% or higher N2 is desired. If unable to obtain 20%, use of max motoring to a minimum of 15% was the acceptable RPM for selecting start lever to IDLE/Rich position. With an unserviceable N2 gauge, start lever to idle was positioned at 3% N1.
The CF6-50 min N2 is also 15%
LEM,
But actually, unless you have some damage, if the rotors are free, you won't get an N2 that low.
You can get a low N2 for various reasons but mainly due to low pneumatic pressure, i.e. APU U/S and having to use ground air cart for starting, start valve partially opening or defective starter.
Motoring engine to below 100 deg, EGT before start lever to idle, was SOP in our operation even if we achieved the required start RPM.