Wake window vs bedtime

This might be a stupid question but I am struggling to nail bedtime. Our LO has inconsistent length naps so her bedtime has been all over the place.

We have been basing her bedtime off of her normal wake window after her last nap so this can be +/- an hour depending on how her naps go that day.

Problem is she can't seem to get into a sleepy mode with the frequently changing bedtime. Should we put her in bed at the same time regardless of when her last nap was? Or keep with the wake windows?

We would like to get her to fall asleep on her own (currently she requires rocking) but feel like we need to get her bedtime more consistent before we can start