If you want to slow yourself down with your fins while fishing, you can't really maintain a 45-degree position relative to the shoreline (assuming you want to keep a contant distance from the bank). Your fin kicks will back you away from the shore if your boat is pointed slightly at the bank.
If I need to keep a constant distance from the shore (like while pounding the banks with streamers), then I keep the boat facing directly downstream and I kick to slow myself down. I twist slightly at the waist and cast off to the side. It takes a bit of practice, but you'll get used to it.
If you're drifting nymphs under an indicator, you really just need to find the right current seams, so constant distance from the bank is not always a necessity. But you do want constant distance from the seam, so you don't have to continually adjust how much line you have out. In this case, you just need to be aware beforehand where you want the boat to be to drift the nymphs in the correct spot. Set up well in advance and you'll be okay. But again, you'll likely have to point the boat directly downstream for the entire length of the current seam if you want to kick to slow yourself down. If you don't care about drifting by the prime lies quickly, then don't worry about kicking, just point your boat however is most comfortable for you and drift along. But I like to constantly slow the boat down by kicking, so most of the day my boat is pointed directly downstream.