bit late in the day now to gain weight if they were on a good poultry fattener/growner at 20% plus roaming and given apples, veg etc then they should be ready to go now close to 20 weeks. The finisher is usually corn/cracked wheat from mill or farm feeds I think given a few weeks before just to get good golden fat covering. Its been a few years since I raised them and cant remember the full details. Assuming you are killing at around 18th December you have 4 weeks left. If you can weigh one now and halve the weight this gives approx oven weight.