The file with frequency values for the advanced page with miner settings is
/usr/lib/lua/luci/model/cbi/cgminer/cgminer.lua
Look at the end of the file for the frequency list.
Here are highlighted in red the lines with new frequency not included in the stock firmware.
pb:value("13:300:0b82", translate("300M"))
pb:value("14:293.75:1706", translate("293.75M"))
pb:value("14:287.5:0b02", translate("287.5M"))
pb:value("14:281.25:1606", translate("281.25M"))
pb:value("15:275:0a82", translate("275M"))
pb:value("15:268.75:1506", translate("268.75M"))
pb:value("15:262.5:0a02", translate("262.5M"))
pb:value("16:256.25:1406", translate("256.25M"))
pb:value("16:250:0982", translate("250M"))
pb:value("16:243.75:1306", translate("243.75M"))
pb:value("17:237.5:1286", translate("237.5M"))
pb:value("17:231.25:1206", translate("231.25M"))
pb:value("18:225:0882", translate("225M"))
pb:value("18:218.75:1106", translate("218.75M (default)"))
pb:value("18:212.5:1086", translate("212.5M"))
pb:value("19:206.25:1006", translate("206.25M"))
pb:value("20:200:0782", translate("200M"))
pb:value("20:196:1f07", translate("196M"))
pb:value("20:193:0f03", translate("193M"))
pb:value("23:175:0d83", translate("175M"))
pb:value("27:150:0b83", translate("150M"))
pb:value("33:125:0983", translate("125M"))
pb:value("40:100:0783", translate("100M"))
Many thanks, just made these changes to my S3+. Very useful information, kudos to you for providing it!