The Vol specification mentioned in the datasheet is the worst case value at a particular current. This worst case value has added margins so that the expected device performance is guaranteed. Thus, actual Vol on the device will be lower than this Vol (max) for that particular current. In order to estimate Vol at different currents, you need the Input/Output Buffer Information Specifications (IBIS). The IBIS data is meant to represent the performance of the actual devices and is correlated to the device SPICE models. IBIS data is calculated for process, temperature, and voltage variations.
For example, consider the POWR1220AT8 IBIS (make sure you are logged in to lattice web account to access this link) data, for the signal name OUT6 it uses model cmos_od50. Now look for model cmos_od50 charactersistics table in the IBIS data file. You can find Pull Down voltages at various currents for this model. Plotting a graph using these data points would help you in estimating Vol at various currents.