Doing Water Change- Need Help ASAP

[Jaws]

So I'm doing a 40G water change on my 180G tank. I've had the salt mixing for a couple days now. I noticed tonight that when I went to do a water change there was a film on the top of the water. I tested the params and they're all a little low but they don't look too bad.

Ca 330
Alk 8.0
Mg 1175
Salinity 1.021

Now I was at then end of two buckets of Kent salt that I combined to make this mix. Both buckets were totally fine in previous water changes. I was about two cups short on salt which shouldn't make too big of a difference since my salinity is 1.025 in the main tank. Should I use this mix? What do you think?

I've got supplements to bring the levels up to where they should be.