Correct temperature and technique for frying in cast iron
There are a few things to keep in mind when cooking with cast iron:


It must be seasoned properly. The biggest mistake people make here is to have the oil too thick when seasoning. You need to wipe the cast iron with a dry paper towel to remove as much oil as possible. If your pan feels sticky when clean you've used too much oil and should start over.
Yes, you want your pan hot before adding fish (see below). 
You want your pan evenly heated. Cast iron is a very poor conductor of heat and it takes a while to fully heat evenly. You're best heating it a lower flame/setting for a longer period before putting in your food.
A little oil rubbed on both sides of your food will help. 
The advice you got about letting the fish release is 100% correct. That's a common mistake people make. As you found, it works like magic. Some things, like eggs, will always stick and are best for a non-stick pan rather than cast iron (even really well-seasoned cast iron). 
In terms of temperature, I would suggest maybe 400-425° for the flesh side and 450° for the skin side. I generally cook the fish first on the flesh side long enough to leave the perfect color marks and then cook mostly on the skin side. If you score the skin it will be crispier (and salt also helps). A temp gun is really useful, and I highly encourage you to purchase one, but flicking some water on the pan will also give you a pretty good indication of how hot it is. If your water evaporates slowly it's too cold. If it evaporates quickly then the pan is hotter, but not hot enough. If it rolls around the pan and takes some time to evaporate, then your pan is hotter still (at least 380°F), and probably about the range you want. The reason it takes longer to evaporate when really hot is that steam under the water droplets is insulating the droplet from the pan, causing it to take longer to evaporate. (This is called the Leidenfrost effect.)