In the 21.1 Infinite Flight ATC Manual, 6.10.5 gets an updated on the use of “reasonable” when issuing an ILS clearance without vectors.
6.10.5 — Controllers may issue an ILS approach clearance at an appropriate position without an assigned intercept heading if the aircraft’s filed STAR and/or Instrument Approach meet one of the following criteria:
- contains a reasonable* intercept angle to the final approach course or,
- joins the extended final at a distance of 20nm or greater
In all cases, an appropriate altitude must still be assigned to serve as the lowest authorized altitude until the aircraft is established on the localizer. If a Controller has an advanced understanding of an airport’s specific ILS approach plate, the Controller may still issue a clearance without an intercept heading outside of the two conditions listed above. To do this, the Controller must have familiarized themselves with the instrument approach procedure and be able to visually determine when aircraft are established. If in doubt, Controllers should opt for issuing vectors instead (see 6.10.3 above).
The addition being the asterisk and this explanation as to why there is one.
**reasonable is considered anything that resembles the usual intercept angle of approximately 30 degrees and/or allows aircraft to turn onto final without difficulty*
If that criteria can not be met then you’d need to give them a normal clearance heading and altitude instruction.