Vet doing talk said the feeding guides are based on research using entire active dogs and when lose those sex hormones by spaying/neutering should cut food back by 25%, I was surprised it was so much. Easiest to weigh out the daily portion (can then split how you like) as by scoop can vary calories significantly.
I always take bag RDA as just a starting guide as activity/metabolism is so variable find its better to assess dogs body score regularly by eye and feel along with weighing every so often (vets/pet stores for big scales) so can alter feed accordingly and track changes before becomes significant. The cheapest way to improve dogs health & reduce future vet bills is to keep them slim.
Millies is good food and if contact customer service many brands will advise which product will be most suitable and may send samples to try. I've used Markus Muhle and Lukullus (zooplus/Bitiba own brand) cold pressed which scored well on All About Dog Food site, sure both have non chicken versions but not checked the finer ingredients to know if 🐔free. They're compressed into crayon shapes so it actually looks less by eye than same weight of kibble. Smelled nice to my nose & dogs enjoyed it, I liked that CP doesn't absorb water and swell up loads which is what happens to kibble in gut but it's less handy to use daily portion as training treats and wouldn't fit in some of my activity feeders.
If she'll eat it you can also add veg to meals to bulk them out with fewer calories, I mainly used green beans as low sugar and easy to get big frozen bag cheaply, but also cauli, brocolli. kibble that's been pre-soaked also increases in volume and aids hydration but some dogs dislike the new texture. Splitting meals helps to feel fuller over day and avoid hunger pukes and feeding with activity feeders slows eating along with enrichment.
Senior dog food is usually less calories than adult as assumed lower activity levels and bulked so can feed a more normal portion size than the adult version. Can check with manufacturers advisors if would be a suitable swop nutritionally for your dogs needs
My dogs appetite increased due to medication side effect into a whiny hungry hippo so I had to employ all these tricks in order to keep her lean & my sanity intact. Vet nurses run weight clinics so useful source of tips if find you now have a dog that's more effort to keep trim than before.