Theres a few free ones about , Stars by stelrad seems to be a popular one, theres the mr combi ones,
strangely I usually go back to doing it long hand like we did at plumbing school.
Just do what every other monkey seems to be doing
Chuck a 28kw combi on the wall with a couple of 4ft doubles in the living rooms and 4ft singles in the bedroom
It is surprising how small the rads can be in some of the new builds now, especially where they have been sized correctly.
Trouble comes with replacement rads, where the customer doesnt want to redecorate so you fit to match the existing span, but then they also demand double rads. Two weeks on they complain it's too hot 🙂