I must admit that I only use my stomach for injections, rotating around across the whole front from side to side. I don’t pinch at all, according to my DSN that is now old advice and not needed with the shorter needles in use today. I am using 4mm needles currently. I’ve never injected in my thigh but I know others use alternative injection sites and may be better able to advise you.
As for adjusting your insulin, as
@Bruce Stephens says, best to ask your healthcare team about that. You don’t say what you’re using, basal & bolus or a mixed insulin once a day, etc. so it’s hard to say. But generally your DSN will suggest adjusting a basal or mixed dose by 2 units for a couple of days, testing to confirm how it is working for you, then repeating if needed to gradually increase the amount of insulin to suit your body. For bolus doses you will be given a ratio to use, number of units per gram of carbs, so that is a matter of adjusting to match what you are eating.
Good luck with everything and keep asking questions, especially of your DSN to build your knowledge.