Generally, this seems to be very useful. I hope the math is correct (havent verified it). The keyboard/numpad hides the Calculate buttons at times, so I wish the back/calculate buttons were at the left/right top instead of bottom. Just seems like some user-interface enhancements are needed to an otherwise-useful application.
The only difference between the free version and the pro version is the lack of advertising in the pro version. Carelessly, they left the link to "Pro edition" in the Pro app! So, there is basically no difference between the apps (with iOS9 controls). At least for $5, I wouldve expected some branding to indicate its the pro version.