I make quite sketchy landings. A big part of that is I understand the basics of traffic patterns, but don't know when to turn. 

Much appreciated,

-Spyhawk

Hmm, here's what I do, though it's more associated with flying jets. Take your shoulder and draw a 45 degree line from it. When the threshold is 45 degrees off of your shoulder looking aft, start that roll to base. I'm used to a curved or half circle turn from downwind to base. At any rate, I like to say, half way round, half way down. So if you are flying it squared off, you want to be half way down mid base. It's easier in VR/track IR to get a good feel of roll out onto final. The key is having good displacement from the runway on downwind. Again, this is for jets, but may work for you. Take your nautical mile per minute squared and divide it by 9. Lets say we are doing 100kts on downwind. 100kts divided by 60 is about 1.6 miles per minute. 1.6 x 1.6 is 2.56. 2.56 divided by 9 is .28 miles of displacement. That is based on a 25 degree bank through out the turn. If you want to fly the downwind at 100kts, then you want about a half mile of displacement. Just remember that winds can impact your ground speed and under shooting/over shooting winds can impact your displacement. You don't have to be exact, but this technique will give you a baseline. The smaller and slower the aircraft you are flying the less impactful your numbers will be. It means you can easily make adjustments when flying smaller/slower aircraft.

The 45-degree method you describe is the same as I was taught in a 172 when I learned to fly.