The time predictions is not that reliable. Also that can be easily achieved with some lines of gcode. So you can write your own custom command for preheat or put the same code as start code in your slicer configuration (would be the normal way).
Simply heat bed to target - x with wait, then heat to target without wait and start heating extruder with wait, then bed with wait.
After that both have target temperature with extruder having started midway of bed heatup.