Horizontal pipe won't add to static head pressure, it will add to total head pressure from frictional forces only. Friction is directly dependent on the velocity of the fluid so if the flow rate is kept low enough for the pipe size, horizontal runs will add negligible head pressure since frictional forces are low. This is whole purpose of using a larger pipe size, to reduce fluid friction. Same goes for the drain, with flow rate and pipe size equal to the return you won't have any friction on your horizontal runs either, slope for drainage is a good idea but no harm will come from large horizontal runs. You're just adding more volume that will require consideration for outages.
Also note when you use the head loss calculator choose a pump that results in the flow rate you're looking for given your tank size. This will give you an accurate total head pressure and flow rate to aim for when selecting a pump.
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