I've been playing quite a bit lately with advance curves, and have been doing some reading on the subject. For performance applications, you might choose something quite a bit different than stock. When I got my cam, it was recommended that I go with a static advance of 20 degrees. I thought that was a bit much, but a number of sources confirm that long duration cams may prefer it. Mine seems quite happy with it. The other thing I was told by the cam supplier was to get to full advance by ~3,500 rpm. I was initially targeting in the 30 - 32 degree range total advance but based on some of the information I found, up to 38 to 42 degrees may be advantageous for low compression engines (8.5:1) like the X. Right now, I've got mine set for 20 degrees static (and up to 1,000 rpm) with ~39 degrees above 3,500 rpm. It will quickly wind the tach off scale if I let it with no evidence of pre-ignition on regular gas. If you are running premium, you might go for a few more degrees as it burns slower.
I've been experimenting with the curve on my Ducelier distributor (1974) and it is not too difficult to adjust the curve other than having to pull the distributor out every time (I've gotten pretty quick at it) since I don't have one of those Sun machines. I plan to replace it with a Bosch distributor out of a 79. Unfortunately, the carb Bosch distributor for the X has way too much advance and too late. The FI cars got a more suitable curve. If anybody has recurved a Bosch I'd be interested in the procedure and the availability of springs, weights, and advance stops for that model.