Hi Peter,
if you want to find the exact range for all the t-codes you should know the all t-codes available in the System. Please follow the below steps.
1.Get all the t-code in the system with the help of table TSTC (this will give the list of all t-code in aSequential manner so this will give exact series).
2. from the list find out Inclusion and exclusions. Thats is you maintained t-code range.. in S_TCODE.