Ooh, I love Super 8! Back in the '90s I'd use what little money I had to buy K40.
Anyway, first thing: you can trust your iPhone meter within reason. 1/50 is roughly a 180 degree shutter at 24fps. So in that case, you'll be shooting wide-open at 24fps. (18fps will slightly overexpose the negative, which is no bad thing).
The cartridge is for tungsten balanced film, so it will engage the orange colour correction filter for shooting outdoors. In that case, your effective ASA will be 320. (Edit: it seems that the Vision 3 cartridges are not notched, but in your case, thankfully it doesn't matter). You did the correct thing by deactivating the filter for indoor use (so that the ASA is 500).
Indoor lights are now often daylight balanced, which might confuse things a little, but it doesn't matter too much. The most you'll have to do in post in that case is to correct the slightly cool colour cast. Not a really big deal.
Conclusion: no need to push, assuming that you metered correctly with the iPhone.
Footnote: some people recommend overexposing 500T by up to 2/3 of a stop in the Super 8 format, simply to reduce excessive grain. This is not necessary for larger formats. In your case, given 1/50th and f/1.4, the only thing you could have done to overexpose is either use a larger shutter angle than 180 (the XL cameras have a 270 degree shutter IIRC); and/or set the frame rate at 18. Some cameras had f/1.2 lenses, but their zoom range was limited. I'd take the wider zoom range over the wider aperture any day.
Edited by Karim D. Ghantous, 22 January 2018 - 03:14 AM.