There is a company out there called NextGenSystems LLC that makes a ton of airplane weight and balance apps. You can find their stuff all over thr app store. I think they will customize or make an app specific to your type of plane if they don't already have it. I used one of them once for a former company and it was pretty awesome. I think this is their website
Nextgen Systems - iOS Apps for Aviation
I think I paid 2.99 for an iPhone app back in the day and it was totally worth it