Boba foot rests, when to use?

[erigeron]

My girl is almost 9 months old and on the larger side for her age, but not huge. When I wear her in the Boba, the carrier body comes within 1/2" of the back of her knee. At what point am I supposed to start using the toddler foot rests? I don't want too much of her thigh hanging out of the carrier, I'd think that would be uncomfortable for her, but I don't think we're there yet.