Velocity factor can vary, even along one roll of twinlead from the manufacturer, if the plastic characteristics are not well controlled. With foam types, it depends on how much air is bubbled into the dielectric. So I always assume that I’m going to need to do some adjusting to find the correct tap points.
And, of course, part of that is having built my first J-pole (for 15m) before internet calculators, or even the internet, for that matter. I just got a roll of twinlead from Tandy / Radio Shack and experimented. Data sheets? The ARRL Handbook gave a nominal value of 0.82 - 0.85.
Yes, it’s great to have a good calculator, but if it doesn’t come out quite right, don’t be afraid to experiment with the stub length (bottom shorting bar position) and the coax tap point to improve the match.