We do not cap your available space so we can not calculate a value for percent of available space remaining. Data storage is billed at $10 per gigabyte, but there is no configured maximum value.
You will not experience any messages or restrictions saying you need to purchase more space when you reach a point of requiring another gigabyte of space. Instead your table(s) would be saved, your storage space utilized would increase, and your bill would increase if warranted.
A live measurement of your current space utilization can be found in Tool #1980 Library Dashoard. There is also a daily record of current space utilization captured and stored in the LIBSIZE table, location FILExx (xx = your credit union ID).