Set the vocal level in the house, then add vocals to the monitors.  Obtain a reasonable balance and level, then have them bring up their instruments on stage.  Any and all amplifiers should be pointed towards the players ears, not out at the audience.  Tilt-backs or amp stands will help.  If and when they cannot hear the vocal monitors, turn the amps back down until they can rather than boosting the monitor level.  If they can't accept this, get new musicians.

 A 50 Hz wave is also about 22.6 feet long. (To calculate how long an audio wave is, divide the speed of sound—1,130 ft./second—by the frequency. For a 50 Hz wave, 1,130/50 = 22.6 ft.) 

Standard reference calibration is one of the most common calibration methods because it is the least subjective. The goal of this method is to ensure that when the output meters on the mixer register 0 dB, the SPL in the audience is a specified decibel level.

This level will vary depending on your application:

Acoustic (folk, spoken word, etc.): 75 to 90 dB
Jazz: 80 to 95 dB
Classical: 100 dB
Modern house of worship: 90 to 95 dB
Electric (rock, country, R&B): 95 to 110 dB