How to set downlink channels for US902 compliance for Packet Forwarder?

How does global_conf.json establish the downlink channels?

US902 compliance requires downlink channels to follow this convention:
8 channels numbered 0 to 7 utilizing LoRa 500 kHz BW at DR8 to DR13, starting at 923 MHz and incrementing linearly by 600 kHz to 927.5 MHz

In global_conf.json this should be specified the following way shown below.

However, the values for “if”:-600000 and 600000 are not allowed by this gateway. When given those numbers, the gateway will simply not reboot after a “Save and Apply”. It will take the numbers 0, (+/-)400000, (+/-)200000 just fine. But it is clear from documentation that the downlinks are separated by 600 kHz.

What is the fix, or am I missing something?

"radio_0" : 
		{
			"enable" : true,
			"freq" : 925100000,
			"rssi_offset" : -162,
			"tx_enable" : true,
			"tx_freq_max" : 927510000,
			"tx_freq_min" :  923299000,
			"type" : "SX1257"
		},

Along with

"chan_multiSF_0" : 
		{
			"enable" : true,
			"if" : -1200000,
			"radio" : 0
		},
		"chan_multiSF_1" : 
		{
			"enable" : true,
			"if" : -600000,
			"radio" : 0
		},
		"chan_multiSF_2" : 
		{
			"enable" : true,
			"if" : 0,
			"radio" : 0
		},
		"chan_multiSF_3" : 
		{
			"enable" : true,
			"if" : 600000,
			"radio" : 0
		},