If your feeding the Senior as just a treat than that's fine. If you're feeding it as a vitamin and mineral supplement, you're not getting much value out of it. The product is "diluted" with forage products so it's going to take even more of it to get a sizable amount of them. If the free choice product your offer isn't going it's job, than you need another products. It should have less than 20% salt, twice as much Ca than P (20%:10% is you feed mostly grass hays) and 100,000 IU/# Vit A.
Another problem with only feeding 2-3# of senior a week is the bag will go bad before you use it up. Not now unless mice get into it but in the summer for sure. It's also not going to add any calories to her diet to help keep her weight up. If what you really need is a treat plus a vitamin/mineral supplement, I'd go with a product like Nature's Essentials. It's got a much better vitamin and mineral profile that could do some good even with erratic feeding.
As a comparison 1# Senior will have 3500 IU/# of Vit A while the NE will have 24,000.