You can do the rounding of the value directly in the ui_text node with something called Angular directives.
{{ msg.payload.Voltage_L1_N | number:2 }}
Here is an example based on the previous flow and also an example Function that prepares an Sql Insert query that can be wired to an mysql or sqlite db. It can be used to insert the data in a Table called Volts and as long as you have prepared its Columns to be dt for date timestamp, Voltage_L1_N, Voltage_L2_N, Voltage_L3_N for the respective modbus values.
[{"id":"050ac9e17b369797","type":"ui_text","z":"54efb553244c241f","group":"1d36cefd3fc95bb2","order":1,"width":0,"height":0,"name":"","label":"Voltage_L1_N","format":"{{ msg.payload.Voltage_L1_N | number:2 }}","layout":"row-spread","className":"","style":false,"font":"","fontSize":16,"color":"#000000","x":800,"y":3120,"wires":[]},{"id":"c578b60831dd386f","type":"ui_text","z":"54efb553244c241f","group":"1d36cefd3fc95bb2","order":2,"width":0,"height":0,"name":"","label":"Voltage_L2_N","format":"{{ msg.payload.Voltage_L2_N | number:2 }}","layout":"row-spread","className":"","style":false,"font":"","fontSize":16,"color":"#000000","x":800,"y":3180,"wires":[]},{"id":"c45e18453f0fbebc","type":"ui_text","z":"54efb553244c241f","group":"1d36cefd3fc95bb2","order":3,"width":0,"height":0,"name":"","label":"Voltage_L3_N","format":"{{ msg.payload.Voltage_L3_N | number:2 }}","layout":"row-spread","className":"","style":false,"font":"","fontSize":16,"color":"#000000","x":800,"y":3240,"wires":[]},{"id":"17b04ff71b9b3717","type":"modbus-read","z":"54efb553244c241f","name":"Voltage","topic":"","showStatusActivities":true,"logIOActivities":false,"showErrors":true,"showWarnings":true,"unitid":"","dataType":"InputRegister","adr":"19000","quantity":"12","rate":"2","rateUnit":"s","delayOnStart":false,"startDelayTime":"","server":"9782f823cc54f17a","useIOFile":false,"ioFile":"","useIOForPayload":false,"emptyMsgOnFail":false,"x":270,"y":3160,"wires":[["92737732fdc11aa6"],[]]},{"id":"92737732fdc11aa6","type":"buffer-parser","z":"54efb553244c241f","name":"","data":"payload","dataType":"msg","specification":"spec","specificationType":"ui","items":[{"type":"floatbe","name":"Voltage_L1_N","offset":0,"length":1,"offsetbit":0,"scale":"1","mask":""},{"type":"floatbe","name":"Voltage_L2_N","offset":4,"length":1,"offsetbit":0,"scale":"1","mask":""},{"type":"floatbe","name":"Voltage_L3_N","offset":8,"length":1,"offsetbit":0,"scale":"1","mask":""}],"swap1":"","swap2":"","swap3":"","swap1Type":"swap","swap2Type":"swap","swap3Type":"swap","msgProperty":"payload","msgPropertyType":"str","resultType":"keyvalue","resultTypeType":"return","multipleResult":false,"fanOutMultipleResult":false,"setTopic":true,"outputs":1,"x":470,"y":3180,"wires":[["050ac9e17b369797","c578b60831dd386f","c45e18453f0fbebc","5b47032292f14787","c9d76cdce7e3eb1b"]]},{"id":"5b47032292f14787","type":"debug","z":"54efb553244c241f","name":"debug 60","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":565,"y":3120,"wires":[],"l":false},{"id":"c9d76cdce7e3eb1b","type":"function","z":"54efb553244c241f","name":"prepare SQL","func":"let Voltage_L1_N = msg.payload.Voltage_L1_N\nlet Voltage_L2_N = msg.payload.Voltage_L2_N\nlet Voltage_L3_N = msg.payload.Voltage_L3_N\n\nmsg.topic = `INSERT INTO Volts (dt, 'Voltage_L1_N', 'Voltage_L2_N', 'Voltage_L3_N') VALUES (datetime('now', 'localtime'), ${Voltage_L1_N}, ${Voltage_L2_N}, ${Voltage_L3_N})`;\n\nreturn msg;","outputs":1,"timeout":0,"noerr":0,"initialize":"","finalize":"","libs":[],"x":790,"y":3360,"wires":[["1c096f3f80da4000"]]},{"id":"1c096f3f80da4000","type":"debug","z":"54efb553244c241f","name":"to DB node","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":980,"y":3360,"wires":[]},{"id":"1d36cefd3fc95bb2","type":"ui_group","name":"Voltage","tab":"647eed3084182c30","order":1,"disp":true,"width":"6","collapse":false,"className":""},{"id":"9782f823cc54f17a","type":"modbus-client","name":"Janitza_Test","clienttype":"tcp","bufferCommands":true,"stateLogEnabled":false,"queueLogEnabled":false,"failureLogEnabled":true,"tcpHost":"172.20.44.18","tcpPort":"502","tcpType":"DEFAULT","serialPort":"/dev/ttyUSB","serialType":"RTU-BUFFERD","serialBaudrate":"9600","serialDatabits":"8","serialStopbits":"1","serialParity":"none","serialConnectionDelay":"100","serialAsciiResponseStartDelimiter":"0x3A","unit_id":"1","commandDelay":"1","clientTimeout":"1000","reconnectOnTimeout":true,"reconnectTimeout":"2000","parallelUnitIdsAllowed":true,"showWarnings":true,"showLogs":true},{"id":"647eed3084182c30","type":"ui_tab","name":"Power analyser","icon":"dashboard","disabled":false,"hidden":false}]
