I'm no expert, but I recently replaced the sand in my filter and downloaded the filter manual for instructions. 3/4 sounds about right, however, I believe you measure (at least I did) by pounds of sand. You should see that on the filter label or go the website for the filter manufacturer where you should be able to find the filters sand capacity. Unless its 10 years old or you have a faulty valve you should not have lost a lot of sand while backwashing. Why did you open the filter to see? Is there some other problem your trying to solve?
As long as you added the recommended amount, everything is fine The sand filters need a little room for the water above the sand to work correctly - if the filter was full to the top, it wouldn't function properly.
Have a beer and hop in the pool - no need to stress yourself over this