When using HowMuchPhe in Safari, at one point I could perform simple inline calculations when inputting quantities eaten. When typing the amount eaten, I could type 4.0+7.5-3.4= and the predictive text feature would calculate 8.1. I could tap that to switch the text to 8.1. This prevented me having to constantly flip back and forth to the calculator while logging food and sped up the process significantly.

This might require changing the input type and having to check that the input type is numeric when a user clicks Add To Log.