The antibiotics are mostly to select for transformants as the inserted DNA has a resistance gene to act as a marker.

I think the question is more about the antibiotic selection for the plasmid in the bacteria, not the resistance cassette in the t-DNA. Omayra, is that correct?

If so, the selection is acting to keep selective pressure on the bacteria to keep the plasmid so the plasmid can then be transmitted to the plant cells via t-DNA integration. If there were not antibiotic selection, then the bacteria will eventually lose the plasmid and your plant transformation won't be very efficient (if it happens at all).

I don't know if there is a direct mathematical link to antibiotic selection level and transformation efficiency.