Hi 🙋♂️
The stretch can be estimated based on the current size. It's important to know whether you're growing more Sativa-dominant or Indica-dominant strains. Indicas typically grow about twice as big as they are at the time of the flip, while Sativa-dominant strains can sometimes reach 1.5 times that. For width, you just need good training.
To your original question: yes, I think you should slowly start the flowering phase. They should be able to fill your tent.
Good luck moving forward 👊👌🍀