I ordered a set or Formatts after researching them on RedUser extensively. I would have to warn about getting a bad batch. I ordered .9-1.8. I saw noticeable image tint and degradation as you ramped up to 1.5, and the 1.8 was SO OFF, I had to balance to 10,000K to get a grey card to match, not to mention the tint. I'm sure there are some that work great, but I would try to test the set you receive specifically before you walk away.
I sent mine back to Formatt for a quality check. They agreed to perform a quality check and covered the shipping to the UK, but they are taking awhile to get them back to me. I have jobs passing by that need them.
Are these IR NDs? If not, you will definitely get weird colours with IR contamination at the higher ND strengths!