I'm just surprise that the car is sitting at 28mm on soft springs at full extension.
I race at Batley and though I am no "set up" expert, my standard kit Yoke yz2 only sits at ~18mm without any need to put limiters on. I'm surprised that a rango wouldn't be similar (as you say, most set ups suggest 19mm so the rtr would usually give a standard set up
Does car have batteries and all the other gubbins in when you measure it, and do you hold it 6inches from ground and allow it to drop and "settle" on a flat surface before measuring?