I think your 1000 GPM might be 1000 GPH, which =’s 16.6 GPM the 300 GPM should be 300 GPH =’s 5 GPM. The pumping rate in GPH, which gives big numbers, is a marketing thing. The Zoeller model 53 does 43 GPM at 5 ft and 19 GPM at 15 ft. The Zoeller estimating tool you made reference to is just that it is an estimating tool and what you get in your basement may be a lot more or less than than that what you estimate.