Alternatively (to @Julian_Kirkness’s suggestion, which is a perfectly one), you could define a {Percent} field with a default value of x%. For those artists where you collect y% or z%, you simply change the value of {Percent} for that row.
It’s a ‘6 of one, half-dozen of the other’ situation — with one exception: If at some point you should change the value of x%, any solution that embeds the value in the formula will recalculate every instance of that formula, including [incorrectly] existing records. Using a default to hold the value of the ‘constant’ makes it persistent to the record and avoids the chance of corrupting historical data.
Note: This approach fails if you create the gig record as one linked from another table. (For instance, if your process has you beginning from an [Artist] record, from where you then select a link that reads “+ Link to a record from gigs.”) Unfortunately, thanks to an Airtable bug, default field values are not honored in records created as linked records. In the past, my solution for that scenario has been to do both, backfilling when required. That is, my formula would read
IF({Percent}=BLANK(),x%,{Percent})
When it came time to change the value of x% globally, you would first need to make sure any BLANK() instances of {Percent} were instead filled with the then-current value of x%, after which you could then change the default value for {Percent} and the default value embedded in the formula.
Again, though, if the value of x% never or very rarely changes, or if you have no need to protect historical records, Julian’s suggestion is easier to implement and maintain.
