L * W * H /231 will yield your numbers
(all in inches and 231 is the number of cu inches in a gallon of liquid)
Each inch of depth in the skimmer section will be 2.3 gallons
45 gallons will require 19" of water depth
Each inch of depth in return section will be 3.1 gallons
45 gallons will require 14" of water depth
So neither will provide adequate volume for a single operation.
I would look at how the detritus settles in the current sump. I would then put my fitting in bottom or as close to as I could the "dirtiest" section. (this is so I could stir up the loose detritus and let it flow out with the effluent.
With this method you will need to do multiple water changes to achieve your goal of 45 gallons.
This would also have only minimal effect on the actual percentage of 10% of your total volume. Otherwise you will have to add drains to the entire sump to drain it.
I have in fact put 1/2 holes low in my sump baffles to allow my single drain to empty the sump with very little effect on how the baffles operate.