Mentor 420cc Moderate Profile Anatomical Implants

I have my breast augmentation surgery booked for a few weeks time with a well known and well respected board certified Plastic Surgeon who has over 20 years experience. For my aesthetic goals and after assessing my skin laxity, measurements etc etc he has recommended 13cm Mentor 420cc moderate profile Anatomical textured implants placed under the muscle. I have grade 1 ptosis and my surgeon believes with the right implant positioned well, a breast lift can be avoided in my case. He would rather not put me through unnecessary costs, surgery and scars if I may not have needed a lift. He said I can always come back for a breast lift at a later date if need be. I guess what I would like to ask is how likely am I to have double bubble or the snoopy/waterfall problem with this particular size and shape of implant. My surgeon says it is always a risk but a small one with the implant he's recommended however I'd love to hear your opinions. Are there any other concerns I should have (I am aware of the risk of ALCL cancer). I am 159cm and 60kg. My measurements are roughly:
Chest Wall: 14-15cm
Fold to Nipple: 7cm
Nipple to Nipple: 20cm

I forgot to mention the following measurements:
Suprasternal to Nipple: 20cm
Edge of areola to inframammarary fold: 5.8cm
So with these additional measurements I would like to see if you think this implant will suit me. I am most worried about the size, that the inplant will be too heavy and cause all sorts of problems and I have read a lot about large implants placed under the muscle causing double bubble.