all about the markets sadly its a lot harder to get an app approved for apple then for android (also why there are MANY useless apps on android) I prefer android due to the highly customizable nature of it being based on Linux.
On the other hand my other half prefers apple due to its ease of use (out of the box) I think my android is easier to use although that's because I have it customized the exact way I like it.
You can always use browser apps (i think) on Apple.
Steam Engine | free vaping calculators has a lot of options but the basic and easiest may be
Ohm's Law Calculator
Making a bookmark to these on your main page makes them as easy to use as an app but will use your data if not using WiFi but since i have no apple help thats the best I can do.
Hope it works out for you its good you are making sure you know these things getting into rebuildables having this resource and an ohm reader are a must.
As far as a ohm tester goes (get a good one, if you don't have one already if you do then the rest of this will probably not be needed.
Although just in case) there are many I cant give any examples of bad or good ones I use have the Segeli version and it seams to work good for me. (although it uses a 14500 battery and IMO they are not very common) I don't even have one so since i primarily build coils at home i just use the provided USB cable. AFAIK the Smok Omnitester sounds nice as it can read your ohms your devices battery output, using a freshly charged battery you can check voltage drop and apparently with both connected you can see your voltage loss with load (something to consider) i bet you can also find a good one that uses simple AA or AAA batteries and give great ohm readings just search online. Sorry for this extra stuff but you need a ohm reader or there is no need for a calculator so hopefully you already have one if not hopefully some of that information will help if not sorry for the large post.
In all I hope this helps you get going.