A 32-bit wide main memory with a capacity of $1$ GB is built using $256\text{M}\times4$ bits DRAM chips. The number of rows per chip is $2^{14}$. The time taken to perform one refresh operation is $50$ nanoseconds. The refresh period is $2$ milliseconds. The percentage (rounded to the nearest integer) of time available for performing memory read/write operations in the main memory unit is ______.