Where a woman has a peerage in her own right, it'll often have a territorial designation to go with it - Isabel Countess of Surrey, etc
In many other cases, the title won't match the surname - Martha (Lovelace) Johnson, 8th Baroness Wentworth
Another complication arises with people like Lady Thatcher, who was never called "Lady Roberts".
Similar issues arise with wives of peers. Especially, it seems wrong to use the prefix with the maiden name.
Likewise, with wives of knights and baronets, it's wrong to use the honorific with the maiden name.
And in all those cases, it's normal in modern practice to use "Lady" + surname, without the first name.
All of which argues against trying to use the Prefix field at all.
It's less of a problem with "Lady" for daughters of earls, but really not necessary, though sometimes they just don't seem properly addressed without it.
Modern Dames in their own right do use the first name - Dame Judi, not Dame Dench. I might put Dame in the Prefix. But then again, if you add DBE, you've got a duplication.