Yikes. Would kill the worms on my stomach for sure...
We use the FlySmart thingy on the iPad and other than taking care of where you're putting your finger it seems like a fairly straight process.
We have two iPads on the cockpit and each crewmember runs an independent calculation which then crosscheck for the same V-speeds and flex. We run two calculations only if it becomes necessary (if there was a LMC). We run the first one just after receiving the loadsheet (after which we need to have matching flex temp and V speeds, if not we crosscheck each other's inputs for errors until said numbers match). If there was a LMC, captain will amend loadsheet as necessary informing "plus/minus X pax" (pax weigh 100kg at our company so it's pretty straightforward) while FO corrects the FMGC numbers (I call my resulting number out loud so he can crosscheck with his number, though it isn't company policy) and after that run the second calculation. Captain will run his second calculation and we will need to have matching numbers again.
If "done by the book" this should be completed by the before start check, though due to the high-cycle/short turnaround nature of our operation, sometimes the last calculation gets done during pushback. But it gets done nevertheless.