I've noticed the same thing. I have also noticed that if I drink too much regular coffee, even with milk, I get a headache. It usually happens at my parents house, and if I am there I probably have not had anything to eat for a while because they never seem to have food at their house. So several coffees on an (relatively) empty stomach gives me a headache.
Between 900-1,000 mg. I use a heaping TBSP of instant coffee per latte and that weighs about 5g. 5g of instant (according to cronometer) has 157mg of caffeine.
I recently added my instant coffee to big a*s sugary skyr fruit smoothies, it‘s an awesome way to get coffee with calories in besides lattes. Btw, I think you were the one that originally turned me on to instant coffee, so thanks for that!