When I first started with nic salts about a month ago, my first batch was the same nic level as my freebase juice, 1.5mg.
1.5mg nic salts felt the same as freebase.
So over the course of about 2 weeks, I mixed 7 or 8 batches of 30ml, incrementally increasing the nic each time a little.
5.5mg/ml of nic salts gave me a near-instant blinding headache, and I was dizzy and had vertigo, even after only 3-4 puffs, so I diluted that down to 5mg/ml and it was fine. However, I settled on 4.5mg/ml because the math was easier to mix in 0.2ml increments with a 6ml syringe for a 120ml batch.
I am now into the second batch of 120ml @ 4.5mg/ml with the nic salts, and I'm likely gonna stay here.
Freebase nic with a level higher than 1.8mg/ml also gives me a wicked headache, but the reaction does not happen for about 20 mins before it hits me, and will persist for hours, however, I found that taking 2x Aleve and immediately chugging about 20 oz of water helps it go away faster.
Getting nic sick is now fun.