I see most people here are targeting dt5 on every radiator.
While this is a good start if you have accurate heat loss figures per room you can mathematically work out the DT per room based on the exact output of the radiators and the heatloss. Most radiators will be slightly oversized and by differing amounts compared to the rooms exact heatloss, so while a standard DT of 5 is a good start it can be improved upon (for the truly COP driven)
While this takes a bit more effort and does require some time in excel it should be easy enough for most people to do.
Feel free to take a look at my spreadsheet attached for my current system and the upgrades (new radiators) (due tomorrow)
It also helps to show system volume and makes it easy to work out both the lowest system design temperature with specific radiators and the lowest temperature the system can realistically run at.
Zak-W_SystemOverview.xlsx (16.4 KB)
Most of the formula is courtesy of Paul Spence from the Heat Pumps UK Facebook group, I just popped it into a pretty spreadsheet.
Negative figures show your radiator is undersized, any DT over around 10 is not going to be too realistic, but you can adjust room temperatures to suit your personal requirements (we use two bedrooms as offices so they are kept at 20°c/21°c
Also another point for the Testo Bluetooth clamps, they are great!!!
Any questions feel free to ask.