When I first started using Fusion I drew lines on sketches in a way that would allow Fusion to automatically constrain the lines. Seemed like a good idea at the time. The problem with that is Fusion would auto-constrain lines to things that were bad choices and that I would have not constrained the line to. So you often get for example, a line that is made perpendicular to some construction line that it crossed, rather than it being parallel to the line it extended from. The result was that later on, especially if I rotated something, the sketch and subsequent parts would go berzerk and I would have a broken and very hard thing to fix. Worse, you might not discover the bad constraining until you got the dreaded failed to compute error, which can be very hard to resolve.
I found that the best way to get constraints right and as intended is to not let Fusion auto-constrain for you and pay very close attention to it attempting to auto-constrain as you draw the line. You can prevent Fusion auto-constraining by deliberately initially drawing lines very crookedly and sometimes un-attached to anything else - so that they begin as blue or orange. Then you manually constrain the line, making it black. It takes more time up front but I feel pays off in the end especially if sketches get more complex. Also make sure that as much as possible of your sketch goes black (constrained) as you go. If you go a long way with lot of blue and orange lines and attempt to constrain it all later, you can sometimes fail to see little flaws that make the whole thing hard to constrain, or over-constrained when you want to add a dimension.
