Hey John. I do this all the time in BeerSmith.
I would convert the 5 gal extract batch to grain first, then scale from 5 gal grain to 1 gal grain.
Your instincts have served you well because you could get a different efficiency from a 5 gal grain batch and a 1 gal grain batch.
For example, I plan 75% for my HERMS Batch Sparge 5 gal batches but only plan 67% in my 2 gal no sparge batches. If you don’t know your efficiency you’re going to have to wing it until you get an idea of how efficient your processes are.
I’d start with 2-2.5 lbs of crushed grain in a cpl gals of brewhaus liquor as a guess. If it’s high gravity you can add brewhaus liquor to thin it out and if it’s low you can add some sugar, honey, molasses, extract, etc to dial it in.