How high should I make an FSK modulation index value?
If the regulations allow you, as wide as your transceiver supports, as long as it doesn't harm the sensitivity. For example, under FCC regulations in the 915MHz band, a transceiver can use ~500KHz for FSK deviation to comply with wide band transceiver regulations. But the same deviation under ETSI in the 433MHz ISM band is not allowed, because its out of band noise will be too high.