According to the manual (https://orgmode.org/manual/Advanced-features.html), named fields in formula should be referred to by $name when a formula is assigned to them
> ‘^’ This row defines names for the fields above the row. With such a > definition, any formula in the table may use ‘$m1’ to refer to the > value ‘10’. Also, if you assign a formula to a names field, it is > stored as ‘$name = ...’. However with 'C-u C-c =' in a named field, the prompt doesn't include the '$' for the name and the formula then defined doesn't work. -- Julien Cubizolles
