Surely you have SOME kind of reference charts in your QRH and/or FCOM! If your computer fails airborne, are you unable to land?
I have done manual performance charts, standalone performance computers, and ACARS-downloaded performance in the 747 Classic and 744/748. In all cases there are/were backups available. Using a standalone computer is NOT an excuse for a failure to have a crosscheck process in place.
You mentioned earlier "an electronic version of the old paper tables". If you have an FMS, you have one. Enter your Flight Plan takeoff weight into the Approach Reference page, and it will give you a Vref for that weight. For the 744, Vref for Flaps25 is within a couple knots of V2 for Flaps10, and Vref for Flaps30 is within a couple knots of V2 for Flaps20. If the V2 from your computer comes up with a lower number, find out WHY! You can certainly come up with a similar crosscheck for your airplane.
You are supposed to be a professional pilot. While you follow standard procedures, you neither follow them blindly, nor become so procedure-bound that you do not understand what the procedures are supposed to accomplish and/or are unable to perform critical crosschecks.