Hi @gnelson what is the use case for having 15 fixed row? When using fixed Repeating group common practice is you know that the data is going to accommodate the fixed cell number.
Alternatively, I would create a repeating group with 1 row and 1 column and for the data source, if you want to keep the limit to 15, is search for thing items until # 15 That way if you Repeating group has less than 15 entries you won’t have that white space.
Hopefully this helps
Cheers