It’s not straight math like 48 x 0.8 or 48 x 0.2.
On most “48V” lithium packs, full charge is actually 54.6V and empty is somewhere around 39–41V, depending on the pack.
So a rough estimate is:
80% = about 50–51V
20% = about 44–45V
Also keep in mind voltage will sag a little under load, so...