A previous comment was made to fill the SMB with air from your BC at your safety stop. If you are properly weighted you will have no air in your BC at your safety stop. At this point, all my BC air is in my drysuit. Sometimes I have a few rocks in my waistband, if I have underestimated weight. Just an example of how all factors are interrelated. Especially the rocks.

I just bought a 10 ft SMB and tried it at my safety stop. I tried to use the BC inflator but the hose would not go on the fitting so I used my alternate. My wreck reel had too much drag and I was pulled toward the surface, remembering to exhale. My momentum carried me to about 5 ft. I later found the BC connector locking ring must be pulled back to get the BC hose on the fitting. On the next dive I used a jump reel with less friction and the BC connector and reel worked fine.
The 10 ft SMB was inflated about halfway on the surface from 15 ft. This is OK and will reduce the chance of a boat running you over if they are looking where they are going. You can surface and finish filling the SMB to show your boat where you are. It is not necessary to fill it completely at depth. I have been told that bigger is better even if you can't use it all at once.