Share of domestic and foreign monetary flows between sectors of a country
The exports and imports in an input-output analysis should correspond to imports and exports as components of GDP so you can use those or some measures derived from them.
For example, trade as a $\%$ of GDP is the sum of exports plus imports over GDP (i.e. $\frac{E+M}{Y}$, where $E$ is export, $M$ import and $Y$ GDP/output).
Hence to calculate any comparable measure from input output table you would have to take the import and export variables from your study (your input-output table should have import row and export column), sum them together and then divide them by output.
You can also find data for $E$, $M$ and $Y$ separately and compare those to the measure you constructed. Based on the question I guess you came to $20\%$ by perhaps doing something like $\frac{E-M}{Y}$?
In addition input-output analysis is not necessary accurate and you should not be surprised to find discrepancies between what your input-output model shows and what holds in real world. Especially, if your input-output model has too many sectors small errors introduced by rounding and linearization can add up and produce results that will not be accurate.