40 lbs does sound like very little sand for a 72 gal
tank. Personally I think twice that amount would "look" better but as mel said it's really up to you and won't hurt anything if it's less.
The following article discuss the advantages of having a deeper "live" sand bed. An Introduction to Deep Sand Beds
If you do want to add then there are two ways you can go about doing it without causing too much of a sand cloud.
First, rinse the sand. Put 10 or 20 pounds at a time into a bucket, and run water into it. Stir it thoroughly, until the water is cloudy. Pour off the water, and repeat two or three times. This will get any contaminants and unwanted detritus out before the sand goes into your tank, and will reduce the amount of fine silt that floats on the top.
Then you can either funnel it to the bottom using a 4" PVC
pipe and place the bottom into your existing sand and lift and place, lift and place till you have a bunch of little hills everywhere which you can smooth out afterwards. This can be fairly difficult but the least messy way to add sand to an existing tank IME.
The other way is to put the sand in large 1 gal
zip lock bags and lower to the bottom before pouring out.