A naked Lookup is returned as an array — and predicting how Airtable will treat an array when used in a formula is definitely a black art. Fortunately, you can always cast a Lookup into a string by appending ‘&''’ – that is, an ampersand followed by two quotes signifying an empty string — which can then be used in VALUE() or similar functions.
You could also define {Appt Finish} as a Rollup field using an aggregation function. To do so, configure it as a Rollup that follows the link given in {Appt type} to roll up the {Nbr of Minutes} [or whatever you call it] field. Then, in the place where you would ordinarily enter an aggregation function, enter
DATEADD(
{Appointment Time},
VALUE(values&''),
'minutes'
)
values is a keyword indicating Airtable should use the rolled up value of the specified remote field. It, too, is returned as an array, so it has to be cast as a string and then converted to a number using VALUE().
Why might you want to do this? Well, in this case you aren’t saving very much processing cost or eliminating much complexity. However, it does reduce your field count by one, in that you no longer need explicitly to look up the ‘minutes’ value and then use it in a formula field, as the remote reference and the formula both take place in the same field.
