As for a pump you could use this
Head Loss Calculator
I personally wouldn't use a flow meter as I'd be concerned about it's accuracy - flood
You think you've pumped 30g but maybe it was 35 and you get wet feet when you go back upstairs.
I have a mark on my sump where the water level needs to be and pump it back to said mark.
You could have marks on your 55g and pump it from 'this line to that line'. Just one initial time use buckets to fill the 55g by hand, making marks as you go. Now you have your start and end points.
Or you run the pump's power cord upstairs and unplug it when you get your sump/DT level where you want it.