That OSG max demand/diversity calculator is a little out of date, all well and good when granny had a couple of three bar heaters chugging away and a few 60 watt incandescent lamps heating the planet up.
Think about what actually runs domestically from a socket circuit.
In bedrooms there's the odd alarm clock, bedside lamp, TV, phone charger, toothbrush charger, shaver socket, occasionally the odd hair dryer and curling tongs etc. So realistically no more than say five amps.
In living areas you have the usual TV, DVD player, possibly computer, internet router, standard lamps, phone chargers etc. Again not a massive amount say around another five amps.
The odd time you may get the vacuum cleaner running around.
Kitchen is where you have your bigger appliances with larger current draw. So kettle, washing machine, dishwasher and tumble dryer. I've left an oven and cooker out of this for now as they would normally be on their own circuits. The appliances rarely run at the same time so potentially your could base your calculation on your highest rated device and add 50% for good measure. Say around 15 amps.
So there you have a basic domestic property drawing around 25 amps at a guess, TBH they're normally a lot lower than that.
This doesn't take into account electric showers, heating, instantaneous water heaters etc.
Maximum demand is really a 30 minute snap shot at one moment in time. You will get the odd spike when the tumble dryer and kettle are running at the same time then Doris decides to do the ironing whilst Desmond is hoovering upstairs, but how often does that happen?
By looking at maximum demand you can then start to work out your diversity and how to balance your circuits for maximum effect.
And then use a bit of SWAG, scientific wild arsed guess...
TBH general use sockets in a two bed domestic property you could get away with a couple of 20 amp radials with a 32 amp RFC for the kitchen/utility room.