Hi David,
First of all well done on producing a good casting,i think
these sort of problems plague would be casters i've had
more that enough over time.
Looking at the video at 1 min 36 the rounded form
may well be gas or even steam from the plaster
pushing back the metal, shrinkage normally has the
appearance of being more collapsed ie being pulled
from the inside ,yours looks pushed from the outside.
If you look at the Myford boy videos he has some very good
castings in nearly all of them the pouring and venting sprues
are extended in height allowing more of a head for the metal.
If you have the aluminium a 750 deg c i think you would be at the
upper end and could try to pour at a cooler temperature as the
sections you have for the sprues and the and the casting are
quite large.
In the photo in album Furnace and casting some small
lost wax castings and injected wax patterns ,aluminium
cast in investment powder flasks with vacuum assistance
as far as i can remember there was a 300 deg differential
between the metal and the flask temperature.
Keep on going your nearly there.
John