GearInchy also has skid patches and gear ratio, which might be good additions to yours.
Thanks for the feedback, GearInchy is indeed fine but as far as calculating ratios goes it only does what the first screen of mine does, and lots of on-line calculators do, which is give you the gear inches for a given combination, so you end up doing lots of calculations and writing the results down. What I wanted was an app that enabled you to find the ratio for a combination you were interested in and then see at a glance what other chainring/sprocket combinations would give you a similar ratio. That's what mine does, and I'm happy for it to do that one very useful thing well and not try to do anything else because I like to keep it simple and focused and easy to use. That's not to say that I won't think about adding other functions such as skid patches, and cats are always cool, but for now I think its adding something new to what's available and well worth 50p (as a developer I'm not too keen on all this free stuff anyway - tends to skew the market in favour of big corporates with deep pockets and against artisan developers like myself).