The easiest way to customize CALC 1 is to simply use standard iOS controls to reorder the calculator list so that you place the calculators you use the most at the top of the list.
When using a calculator, the swipe gesture takes you to the next calculator on the list, So grouping your most used calculators at makes it very easy to change between calculators.
From the calculator list, tap the edit key to put the list in edit mode.
In the edit mode, drag the reorder buttons on the right to reorder the list.
Press done when you are finished.
You can also reorder the Formula List using the same steps.